There are usually discord servers for these kinds of things where people look for other people to play online.
I'd say RPCS3 probably has the best support for this feature, it's as easy as making an account and just going online on the game you want to play. No port forwarding, no 2 hour set up, and the best part is they have a site listing all the games people are playing online live. Usually Tekken Tag 2 breaks the 100 mark and I've seen it go to 150-160 active players. It's honestly pretty impressive that over 100 people on this world would decide to get on an old PS3 fighting game almost daily. It's a pretty nice community.
The 3DS emulators also support a very similar feature where you make an account and just play. I used to play the crap out of Smash 3DS with my friends like that (I was too poor to afford a Switch at the time, but eventually got one and Ultimate) and we also played a lot of Mario Kart, some Tri Force Heroes and I personally went on a lot of people's towns in ACNL on the old Citra.
PPSSPP has been a nightmare to set up though, and I just gave up entirely. I know years ago literally maybe 10 years or more, I managed to get on PPSSPP MHFU online. No damn idea how I did it, considering nothing works now.
Oh and forgot to mention Slippy and Dolphin, those are extremely easy to set up too and I've played a lot of Brawl on there.