Where in the editor can I find the Orcish structures of the Arena? For example, turrets?

Added 7 minutes later

The editor doesn't allow placing a Sovereign in the city garrison. How can I do this?

It's easy. First, place any hero in the garrison that you are given.
Select the city.
In the city properties tree on the left, there should be a section called
GarrisonHero and something like this: AdvMapHero:Maps\SingleMissions\TheGreatDesert\map:item_ABB5AB83-36C2-4DBF-ABD2-A33805F2F519:AdvMapHero
Click on its properties [...].
On the right, the settings for this garrison hero appear. Look for the Shared section, click on its properties again, and change it to Kha-Belekh.
That is, to AdvMapHeroShared:MapObjects\Inferno\Sovereign.(AdvMapHeroShared)
Now you can work with it and configure it even from the city settings menu.