How do you maintain a conversation when you have no idea what someone is talking about?

Tell them you don't know what they're talking about. Either they will explain it to you or they'll change the topic.

Just nod, agree and smile hahaha

Repeat one thing that stood out about the conversation... and add an oh, really, wow, interesting... buys time and - has them think your invested so they further elaborate on the subject...and - fortunately to opt out of it all- you look at the time and mention that you have to go and hopefully you'll catch up on it later

the key is to BS with a good poker face

Well if you don't want to know, just smile and nod. Or frown and shake your head based on the tone of voice. Say "right" or "oh really" or "you don't say." if you want to know you better ask!

You ask questions like you would if you're talking to a child. Say stuff like, ohhh really? Or that's so interesting

Ask them what they're talking about.

nod and smile a lot

Ask .. what are you meaning? If dumbass answer rest is history 😎

Make up some random bullshit