There's a "Search" box at the top of Teams window that has never not found what I was searching for, and - once you've clicked into a channel (including General, which is the default channel in every Team) "Find in Channel" works as needed for me. No grouping is correct but you can use tags to kinda sorta have the same effect.

I'm no big fan of, or apologist for, Teams but this stuff is pretty basic and easily discovered in the UI or documentation.