Hello and welcome to Dakka!
There aren't faction specific forums but there typically are faction specific threads for tactics. These threads are for discussing what works best for a faction with regards to gameplay and the tactics you want to employ.
For example here's one for Eldar:
https://www.dakkadakka.com/dakkaforum/posts/list/450/803950.page
and one for Orks:
https://www.dakkadakka.com/dakkaforum/posts/list/4440/800443.page
If you're looking for specific help on building out an army it's usually best to make a new topic in the
40k army list and ask for feedback. People often post their Battlescribe lists there.
General
40k questions should be fine in
40k General