Horizon of Darkness

Will magic occupy slots based on levels, or will each spell occupy one slot (depending on its strength)? And can the same skill/spell be strengthened/weakened, like in Tyranny (if you've played it)?

Magic works the same way as everything else.
There are no slots based on the difficulty level of the skills.
There are general slots for skills of any difficulty level; their number increases with the character's level. In extreme cases, it can be increased by some items.
Separately, there are charges, the number of which already depends on the difficulty level of the skill and the player's level.
You can create the same spell with different levels of power. Look at how it's done for Revan.

Added after 39 seconds

Does firearms exist in your fantasy world?

Unfortunately, no.