I spent way too much time last week going down a rabbit hole of these on AO3, and the approaches are all over the map. Some writers just drop the Dragonborn into 'The Witcher' with a portal spell and call it a day, but the good ones try to actually weave the metaphysics together.
Like, I read one that had the Daedric Princes interacting with the deities from 'Dragon Age'. It wasn't about who would win in a fight, but more about how Sithis and the Void relate to the Fade. The writer treated both sets of lore as fundamentally true and then explored the friction points—like how a mage from Thedas would react to Tamriel's school of Alteration, which breaks all their rules about blood magic and demonic pacts. The blending happened at the level of magical theory, not just plot.
Then you've got the ones that use a shared 'ancient progenitor' trope, suggesting the Dwemer and, say, the ancient elves from 'Lord of the Rings' originated from some lost common civilization. It can feel forced, but when it works, it creates a strange new backstory that honors both sources. The worst offenders just use the other universe as a backdrop for overpowered characters, but the best feel like a genuine expansion, making you wonder about the connections yourself.
My bookmarks are full of attempts that made me look up lore pages for both series, which is the highest compliment I can give.