Mercenaries come in three different forms within the mod: Clans (companies), Tavern and faction integrated. Much like base bannerlord, the various companies within Estria are clans. You can also recruit Mercenaries from Taverns. What's available depends on your location, since there isn't a generic tree. Faction integrated mercenaries are those that are part of a factions troop tree. The reasons for this depends on lore. Also to make roleplaying easier for players. Whilst not all integrated are strictly mercenaries from a gameplay perspective, the player can treat them as such. Please consult the lore section for more details.
Most companies originate as bands of sellswords from the Riverfell Republic. These bands that gain reputation in battle get the right to be sanctioned under the Republic Charter. Some companies exist which are not sanctioned. These Rogue companies are considered no more than outlaws by Riverfell but are regularly employed by other nations if Official companies are not available. There are currently 4 sanctioned companies and 6 Rogue companies. Sanctioned companies tend to have much higher quality equipment and training but may lack the numbers of their rogue counterparts.
Certain mercenaries are available from taverns. This is mostly to add variety to Estria and to give players more options when selecting the composition of their army. What Mercenaries are available depends mostly on location, although there are some swords for hire who are available everywhere.
Bannerlord uses clans as mercenary companies within the base game and so I've tried to replicate this within the mod. The other ways of hiring mercenaries are for extra gameplay and roleplaying purposes, as base bannerlord is extremely lacking.