Modding

I remember that I used to be able to view models, animations, and textures directly in the editor, separately from the map (there was a button like "edit" when clicking on the resource address in its input field, like with the file names and descriptions of the map, but I'm not sure). Now, in the map editor, I can only change the contents of XDB files, and even then, it's worse than I can do in a simple text editor, and I can't view models with animations, no matter how hard I try - there just isn't a button for it, not even in the context menus.

It's been a few years since I last modded, my hardware has changed, and I've switched to Windows 10, so it might be due to missing plugins or drivers. If you have any ideas about what might be the problem, please let me know.

In the map editor, go to the menu Resource -> Select Tables. You need to select the necessary resources to add to the list.

There are quite a few types of resources; here are some of them:
Model - models.
CharacterView - models with a full set of animations.
Effect - effects.

The selected resource sections will now appear in the list filter; select the one you need and view it.

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However, the new hero completely lacks the effects of the Cave Spawn, and I don't remember which file to look for them in and how to add them.

You probably didn't specify the links to the animation effects.

Effects for an object are specified for each animation (idle00/cast/death, etc.) separately in the < Effect/ > tag.

Here is the AnimSet for Cave Spawn, since you used its model:
Characters\Creatures\Inferno\AlternativeUpgrade\T6_Pit_Spawn-arena.xdb

Here is the idle00 animation:
Characters\Creatures\Inferno\AlternativeUpgrade\T6_Pit_Spawn-arena-idle00.xdb

And here is how the idle00 animation effect is specified:
< Effect href="/Effects/_(Effect)/Characters/Creatures/Inferno/AlternativeUpgrade/T6_Pit_Spawn/T6_Pit_Spawn_idle00.(Effect).xdb#xpointer(/Effect)"/ >

(no spaces in the tag, the forum doesn't allow me to insert it correctly)

Also, the hero uses the same "cast" animation for all spells, even though I also gave him the "buff" animation.

Unfortunately, this is a feature of the Inferno heroes. They only use the "buff" animation for the "Soul Eater" ability; for all other spells, they use "cast".

I also couldn't find the icons for the alternate upgrades of the Inferno creatures in the game files - on the ATB scale and everywhere where the hero icon is used, he now looks like a "Cave Lord" (the regular upgrade).

You can find the icons for alternate upgrades here:
UI\H5A2\Icons\Creatures\Inferno_second_upd