Play as Inferno.

In what order should I build a Castle, capture mines, which creatures should I buy, which heroes should I use to defeat neutral creatures, and how should this happen? Which skills should I develop for each hero, which schools of magic, and why? I want to understand not just follow a certain scheme, but also understand why I am developing skills in this way, for this hero, and in this situation.

You'll have to write a novel for that, for each hero, for each city, on each map.
Moreover, you need to understand the principle - and this can only be done through experience (play more). For starters, here is an article about developing Necropolis (to understand the essence - it should help)
http://heroes.ag.ru/heroes5/advices/11/
And then, by analogy, taking into account the features of the map (sometimes you need to take money instead of experience for someone)

Added 1 minute ago
And in general, read the forum of this site

Added 56 seconds ago
And in general, read the forum of this site