First Impressions

I still don't understand how to assign them; I couldn't find the option either in the city or in the menu.

There's an empty icon in the upper left corner; drag the hero icon from the garrison into it. In general, the interface remains a weak point of the seventh installment. I eventually figured it out, though not immediately. But there are some unpleasant little things, like the inability to access the hero interface from the city interface. You have to exit the city interface and enter the hero interface, even if the latter is in the garrison. From my point of view, the lack of foresight in such small details is absurd.
There's also the encyclopedia. It's a good thing to sit in and read all sorts of Ashan tales... But the problem is that this same encyclopedia is sometimes the only source of game information, not just lore. And, in my opinion, it's less convenient in this role than classic sources of such information.

How to understand "unequal"? Is something overpowered, and something is weak?

No, not necessarily. Some skills are practically a must-have, while others are... well, if you have nothing else to take. Examples: in earth magic, you should definitely take the general regeneration skill first. Then, it's desirable to upgrade earth magic to the next level. Then... Well, you get the idea. But what about the skill that gives +1 to defense for each use of an earth spell? It looks interesting in its own way, but in my opinion, you can only take it as a last resort. If it gave +2 or +3 to defense, I would probably think about it. Because, on average, you don't expect a constant stream of spells from yourself in most battles, and +3 to defense will already make a difference if you cast 1-2 spells. +1 won't make a difference. Do you understand what I mean? And there are many such cases. In the fifth Heroes, the same thing happened in principle, but randomness compensated for this imbalance to some extent. You might not get the skill you need. Or you might try to get a cool skill through several less useful ones, but after taking them, you wouldn't get the cool one you wanted. Here, there seems to be a game mechanic for variability, but in fact, the balance should be much better to achieve this.

I don't know, I didn't notice any obviously useless buildings.

Well, it's the same thing. The buildings seem useful, but if you haven't built basilisks yet, why do you need some not-so-big bonus to something for the heroes around the city? You just don't care about that at the moment.

No, in the sixth part, there was a real mess, where we studied spells for level-up points instead of traditionally learning them in the Guildhall, and putting level-up points into skills.

Ah, well, from that point of view, it wasn't very good. As was the widespread prevalence of healing skills and summoning reinforcements. I agree, it's better now, more interesting.
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It's strange, I can't choose a fort like in the Sixth - to order troops there, etc.

It seems you can't order troops from neighboring buildings to the fort itself. But you can go to one of the cities and make the fort send a caravan with creatures to that city.

And by the way, I think the units from the additional campaign about Axoth (White Tigers, for example) are not available in the regular game (in Ashan)? I didn't find them in the Encyclopedia - although it has been updated - the Dwarf faction and new neutrals have been added.

I don't know, I haven't played it.