They do missions and sell beast parts/pills/spirit weapons and stuff. They can also make money from properties they own like mines. They also get gifts from people wanting to make connections.
Protection money from mortal kingdoms, Subsidiary companies in mortal world, and grave robbing. There are also some specialized sects that create weapons and pills and sell them. Such sects also own mines and grow herbs in their gardens.
They usually have some people to make pills and artifacts and whatnot, and they have some magic resources like spiritual herb farms, mineral mines, demonic beast corpses, spiritual beast farms--I guess they might sell that (since oftentimes this is the property of the cultivator who made/found it, they might collect a fee from sect members' earnings). They could also engage in normal trade of nonspiritual goods. It's also possible that they collect protection fees, since, as righteous sects, they usually act like vigilantes/police. I suppose they are also big influences in their area and may accept bribes to back political figures and merchants, or do errands and missions for the local bigshots (or just normal people). In the case of demonic sects, they probably just rob/raid, and do black market stuff like human trafficking. I know buddhist monks and temples accept and partially live off of donations, but I'm not sure how daoist sects deal with donations, if they accept them at all. There also those ascetic sects that are pretty much completely self-sufficient and cut off from the mortal world and/or the cultivation world, and thus don't need money. But I feel like these are actually pretty rare in xianxia/xuanhuan.
It's 99% spirit stone mines. It is the hard currency for cultivators plus the tax from the mortals of the kingdom or region where they are based since the sect is supposed to flourish the economy of that region and that supposed region is supposed to be more safer as the territory of ABC sect
They are many industries: hunting, mining, alchemy, farming, artifacts, scrolls and so on. The best thing about wuxia is it's economy.