The Dark Elves seem to have 2 camps with buildings. Plus, near the surface exit, in one of the camps, there is a building that adds 5% to the army each week. In the long run, it multiplies the army several times due to compound interest.
Additional heroes are given in the cities of Humans and Orcs. As soon as you gain 25 reputation with the Orcs, a message will appear stating that a certain hero, who is standing near the border around the Castle, wants to give you a quest. You approach him, take the quest, complete it - and you get this hero and his Castle at your disposal. At 25, 50, and 75 reputation with the Orcs, it works like this; I don't remember about the Humans, but the principle is similar. Many quests overlap with each other. It is better not to rush to kill anyone until you complete the quest. For example, it is better not to rush to complete the quest from the Elves and kill the mine guards before completing the Orc quest related to them. The mines will come under the player's control as the reputation with the factions grows.
Added 4 minutes ago
And it is not necessary to complete every quest and gain 100 reputation. 75 reputation with Humans or Orcs is enough to complete it.
Added 1 minute ago
Before taking quests from the main heroes, it is better to make saves.
Who are the main heroes? Are they the ones standing in front of the cities?
And another question specifically about the quests:
1. Collect flowers for the alchemist. I collected them near the winery and meadery, but I couldn't find the ones in the east. Where are these flowers?
2. Orc mines, where is the abandoned mine in the north?