It was supposed to be Copilot, with access to your Xbox account. So you could ask it things like "what achievements do I have in [game]?" It would also know what game you were playing, and answer questions like, "I'm stuck on [puzzle], give me a hint," etc.
The "on mobile" part was that it would be integrated into the Xbox mobile app.
(I say "supposed to be" because it was just a beta, and hadn't even made it to consoles yet – just PC and mobile.)
Runescape as well. Gold farming for membership bonds severely stratified the game's economy and pushed people towards Oldschool Runescape. From the introduction of bonds until 2017 the number of bots raised at a rate unseen since the switchover to the new skilling menu in 2002.
But this was replaced by Segasages/Gamefaqs. There are a lot of gamers now that will make a YouTube video about it. Seems like a replacement for no reason.
This is such a solved problem though (from my experience). Usually googling the location/quest/puzzle name gets the result you are looking for first time. No need for copilot.
Maybe it's just the type of games I'm playing, for example "how to get nyrulna" in google gives me my answer in the first search result.
I've used those occasionally and they seem to be a mix of official guides and user-submitted video clips, but it's not clear where the user-submitted clips come from.
The "on mobile" part was that it would be integrated into the Xbox mobile app.
(I say "supposed to be" because it was just a beta, and hadn't even made it to consoles yet – just PC and mobile.)