Please help me understand custom maps

I understand that custom maps can be created using forge, but can those maps be played across the online world or only withing a party? Is it possible for me to jump into a custom map game that some random created? How does this work?

The maps are getting a bit repetitive and I know that there are a few custom maps that 343 won’t add to matchmaking (yet). I just wondered if it is possible to play a matchmaking game with a custom map from a random onliner. Thanks.