Online collaborative drawing, for, e.g. doing mathematics online or general collaborative diagrams. Many options here but I have not managed to have a truly satisfactory experience yet, although I have not committed time to this. I suspect that most of the options are going to need some kind of stylus input to be any use, at least for the writing-heavy workflows I care about.
Virtual whiteboard for sketching hand-drawn like diagrams. Collaborative and end-to-end encrypted.
There is a version is with extra work teams features called Excalidraw+ (USD7/person/month).
FigJam is made by Figma who have a very profitable business based on being pretty good at this stuff so maybe that is worth trying.
There are many others. Interactive scratchpad Collusion (USD20/user/month), Bitpaper (approx USD1/page) is a collective sketch page. desmos is an online collaborative graphing calculator. Mural (USD20/user/month), …
There are other options which are offered only on assinine enterprise pricing (e.g. Blackboard Collaborate) which I will not trouble with here.