Firstly: the system is pretty annoying and there is no actual need to use it. The game's description was basically non-existent so I ended up only trying it out after having actually cleared the game and loading my last save... but a few things about it:
1. You're extremely unlikely to get an actually good persona from doing this. At best you'll get an overleved persona with shitty moveset, at least in comparison to what you should be making by yourself. Well it's possible to be extremely lucky too I guess.
2. You don't get coop exp bonus
3. The actual thing to get out of this is skills that can be passed along. Mainly there are three things that are only available through this:
万能ブースタ
万能ハイブースタ
魔導の才能 (+25% damage to all magic)
There is also a fourth skill only available through this, which recovers your SP when you get baton touched onto, but... who cares lol.
Anyway general pointers: sacrificing higher level personas tends to give you higher level personas on average. Sacrificing personas with more skills tends to give you more skills. So basically like the normal fusion system, except you don't know what you get. The game tells you that you somehow fuse personas with random people from online, which probably isn't true. It might like use the data of some random persona someone sacrificed at some point? Whatever.
This system has severe use limitations and gives you mostly shit so how to get anything good out of it? Savescum lol. You can just reload and use another persona if you get shit. If you want to get the exclusive skills to this fusing method, just sacrifice good persona and reload if it doesn't have any of them. It's very luck based, but it shouldn't take that long. I haven't really tried this with low level persona, but I did this at the end of game just going from high level to lower level and I was able to find all three skills by reloading saves for around 20 minutes. Maybe 30.
Alternatively you can use this system to sacrifice persona you've gotten from negotiations or from Ryuuji's coop ability. Usually you'll want to use the useless personas for making other personas, or sacrificing them to level up personas, but since it is a one to one trade you can just do that with whatever persona you end up getting.
This system is dumb, but the boosters are really good (and they stack), so if you want to use 万能 skills it's probably worth doing this.