Look at the license file. They are using their own custom license. Being made available on github doesn't make it is open source. Anytype could be considered 'source available'.
(I specified this just because OP asked for open source. I contribute sometimes to foss apps, and it bothers me that the term foss get misinterpreted. It's not you, even most companies don't know the right terms to be considered foss. If you look up anytype on wayback machine, they used to proudly claim they were foss lol)
As others mentioned, Joplin is a solid option (with lots of plugins too). Notesnook too is good (if you're willing to pay a subscription). If you are willing to wait a bit, there's Beaver Notes. Currently they are working on syncing between devices and a new mobile client.