You can find manually-uploaded recorded games to view, as well as upload your own. Or view the ranked leaderboard to see where you or your favourite pros stand in comparison to other players. There’s also a list of helpful resources like some high-level players’ hotkey files.
It’s created by the AoE4 and (now) AoM streamer FitzBro, who’s streamed a lot of its development on his Twitch channel. The source code is on GitHub.