"ha mm cha" is the noun and "yum cha" is the verb- you'd say "let's go eat ha mm cha" or "let's go yum cha." That is to say, "yum cha" is not a traditional brunch, but rather the act of consumtion. Also, I believe "ha mm cha" would be the brunch, while you can "yum cha" any time of the day, and this type of food would be the kind that you'd get from restaurants that serve dim sum. If you disagree with anything I said, please lmk; I'd love to learn more.
Also a bit surprised with the usage of "boh lo bao" rather than the common English moniker of 'pineapple bun'- took me a second to register lol