Which Pokemon games Nintendo DS / 3DS playable online (fight / trade)?

I'm currently playing Pokemon Perl and have noticed that you can only play / swap / fight with a friend for pearl / diamond via Wi-Fi because Nintendo has discontinued the services for this game. I'm a little confused about what exactly was set and what was not and I also couldn't quite make out of the Internet.

Which games are still online where you can fight against other trainers online or swap Pokemon? I'm a bit late to play the games, but in lockdown I can't avoid it and hope that such services are still active in one of the following games:

Black - White, Black 2 - White 2, Omega Ruby - Alpha Sapphire, Pokemon X - Y? Maybe one of these games? I would be happy to receive detailed and sufficient information and thank you very much in advance.

Only the 3ds games

From X and Y you can play online again. So from the 3DS generation.
The Wi-Fi for the Nintendo DS has been deactivated - you can generally no longer connect to the Internet with the normal DS or games because the service has been deactivated.

In that case that would only be Omega Ruby - Alpha Sapphire and X-Y right?

How exactly does it work then where do I have to go to meet other players?

At this PSS you will find it there

You can connect to Nintendo servers from the sixth generation (X and Y). Nintendo's servers have long been deactivated for DS games, but you can play on private servers. But they are often very empty. But if you really want to do that, you can look for tutorials on the Internet.