You may have seen chat about pbem which is another term for multiplayer. The short answer is, you make a move and send your game files to your opponent; then your opponent makes a move and send files to you.
It is a nice way to play as any human opponent is stronger than AI.
A bit longer answer is this: One player initiates the game and makes a move and saves. Then sends files to the opponent. The opponent makes a move, saves, runs the turn. Then makes a new move and saves.
Repeat the process until done. Usually files are Zipped or Rarred to minimize file corruption and often players chat or banter briefly in exchanging file. It is a very nice way to meet people who share a passion for wargames.
AGEOD games are a bit different than most games because of the WEGO system. Unlike typical pbem play where one player makes all moves and battles and end the turn, then the opponent does the same; in AGEOD games turns resolve at the same time, so the dynamic is a component of pbem.
This forum is a place to find opponents, but several game clubs also provide a place to meet players and start games such as The Wargamers Club which has a very active group with most players chatting in Slack and exchanging files via dropbox.
https://thewargamingclub.online/