The answer is yes, and, no, and sometimes/sorta/depends.

It's all about the planes and the physical laws there. You generally have to work within the physical laws.

However, on planes that contain humans, elves, dwarves, orcs etc. There are certain underlying meta-characteristics that work across the board.

So for example, on an Earth like yours--magic can be done--used to be done. It is a lower Entropy plane so it has always been more complicated/difficult to do Astlan style magic--but there are ways to do it--different spells, different combinations, different rules--but you can do it--particularly animage stuff if you have the mana.

The fundamental problem with "Earth" is that it used to have a lot more mana (and the rest of its localverse generally does). But a few deities and their followers sort of got together (I won't mention their names, but one started out lighting bushes on fire, and another group revolved around a certain chubby bald dude, and then a few cranky ancestors and...) and had/have their priests hoovering up mana at a ridiculous rate, making what wasn't a lot of mana, even less mana.

So, can Tom go to your Earth and use the Rod to access the fire portal and roast an entire town? Yes, yes, he could. It would be expensive and taxing, but he's got a lot of horsepower.

Now, all that being said, there are places in the multiverse where the immortals you and I know, are very hard pressed to even exist without lots of preparation and effort. Of course, these parts tend to have their own immortals, although, oddly enough, as far as we can tell--not that many--or maybe we just can't conceive of or recognize them...

Well, again, it's the physical laws of the plane, where it is located relative to the various polarization forces (see the Appendices of Book III).

And it can be what the gods are doing.

But the multiverse is unbelievably big, and there are sets of laws for some parts of it that are incompatible with "life as we know it" that doesn't mean there isn't life there, but the rules are so far off, it is hard even for an immortal to figure out how to get there safely.

I.e. there are planes where gods must wear special "gear" to exist in those planes. And there are others where they can't. That being said, there is a spectrum of immortals such that some can wiggle into planes others cannot and vice versa.

Think about it like habitable zones around a star. There are certain habitable regions in the multiverse where what we understand life exists, and that region runs a very large gamut in multiple dimensions.

E.g. Entropy vs Order. The more entropy, the more magic, until the point "life as we know it" cannot exist, and the more Order, the less magic, more technology you need up until there is so much order "life as we know it" cannot exist. The vast majority of the "habitable" planes like on the spectrum between those two ends (and there are of course, other polarizations as well)

And a point of matter--pardon the pun--when I say life, I mean material life, in the material planes. TheAbyss, TheOuterplanes, the Intermediate Planes, those exist outside the material plane, so gods can roam pretty freely with their own set of rules. However, different parts of the Outer Planes reflect different parts of the material planes so there is some variation--but this is because immortals want that region of the Outer Planes to reflect different parts of the material planes (hence refleca--btw)

Yada yada the answer is, it varies.

But on top of this, the action of various gods can affect these planes and they can do things that make a world (at least small parts of it) appear more or less entropic.

In the case of your Earth, that "planet" is overrun by mana hoovering priests, so that just makes what was a tough sell for magic, a "very tough sell"

Before the Catholic Church overwhelmed Europe there was a lot more mana there, and in particular shamans, druids, animages could do some interesting stuff. Now there just isn't as much "free" mana laying around, except at certain hidden/remote locations.

There is a reason mystics and monks like to build temples in extremely remote regions of your planet--there is likely to be more mana there. yes it was generated by flora and fauna, not by humans--so there is less than there could be, but still no priests going around sucking it up.

But, if you can bring your own mana--like a powerful off world priest with strong links to a willing god in the Outer Planes you can heal people, and all your other normal priestly spells.

So a Priest of Tiernon on Earth could operate normally--at least in terms of spending mana to do spells--worship would be harder since there aren't any worshipers there to harvest mana from. So, could the Church Tiernon make a move--yes, if Tiernon wanted to spend the effort and take the chance of sparking a war with the pantheons that think they "own" the planet.

This is basically what happened to the Nyjyr Ennead. They were pushed out and their worshipers killed or assimilated (on multiple worlds) They were not helped by that nincompoop Pharoah that lost all the slaves to a certain super deity on the planet. Quite the irony he ends up being an archdemon.

Hmm, possibly, prossibly. My guess would be that their specialization and powers would probably give them the insight to go further into realms that others can't--but that would be more a matter of having the skills/knowledge/insight to get there and survive than anything too.

Most of the "normal" immortals I know are of *noid" type, meaning humanoid, orcanoid, elvanoid, dwarvanoid, monkeynoid, etc. noid. Related to or based on things we know about/see in our normal going about.

Denubians are pretty out there, but they are still pretty mainstream in that they obey similar laws of physics.

The next group of Immortals are still rather normal, even though not necessarily *noid based. They are a bit further out there, and much much much older--more about them in Book 4. They make the Titans look like babies, cosmologically speaking. But I think they are close to the normal range of biology--just a very early version of it.

Now, Tartibsizlik might be someone who go further, and the Knights probably can, both are rather weird. So, yes, thinking on it, I am now sure there are some that can go further, but I think my above statement applies. Either that or they've somehow 'naturalized' with more extreme regimes. Eris is an Olympian so she's what I would call "of common origin" so probably has some limits.

That being said, all their limits, and demon's limits are far beyond what mortals can deal with. What I am talking about is stuff that is literally 'inconceivable' and rather 'unknown' since we can't really explore those parts of the multiverse and much of it is hypothetical. We only have glimpses--some horrifying.

Let's be clear, by 'weird' I mean something much stranger than say the difference between carbon and silicon based life forms. While one would think a significant difference, it really isn't, both can exist in the same localverse, and generally do.