I don’t know when it happened (it seems recent going by the reviews)*, but Microsoft Whiteboard on Android is now available for everyone (personal accounts too), not just for enterprise(?) and education account holders.
Microsoft Whiteboard is a very good online whiteboard for collaboration.
It’s simple, free and intuitive. If you need more features like mind map and templates, I recommend Miro, Mural, Stormboard, Lucidspark and more.