There seems to be four main options I can think of right now.
First would be purely local to-do lists, on Android you have apps like Tasks(dot)org, on iOS you have Apple's reminders app (not FOSS! iOS generally doesn't have many good open-source apps...), on Linux you have lots of options like Planify. If you don't need device sync, these are probably your best option. On Android, Tasks(dot)org also supports Nextcloud device sync (you can use a VPS or a public instance if you prefer) as well as any cloud storage service that supports WebDAV.
Secondly, if you do need device sync, you can use Markdown note apps that happen to tack on to-do list support, like Joplin, Obsidian (not FOSS!), Markor (Android only), etc., most of these are cross-platform. They let you either sync with their own cloud service, through syncing the files (e.g. Syncthing, Git), or with a service like Nextcloud (again, you don't have to self-host on your own hardware if you don't want to)
Third, there are the todo.txt apps. They use a standard "todo" file that is compatible across many clients, so you can look for yourself a bunch of options that support a wide range of different operating systems. This single file can be synced with Syncthing very easily, and the benefit of this approach is that most of the clients are pretty lightweight and aren't too bloaty.
Fourth, you have Vikunja, which is a self-hostable option that does not have a native desktop or mobile client (the Android one is still in early beta with limited features). You access it through a web browser, and it has all the features you could want. I think you can also sync with apps like Tasks(dot)org, but WebDAV support is not perfect and it's a little janky, so I prefer just using the web interface. Vikunja has neat tricks like showing your notes as a Kanban board, grouping them into projects, all that. This makes it closer to something like Trello (that is, for managing projects) rather than a daily to-do list for me, but your use case can vary. You don't need to self-host it either, they offer their own hosted cloud service as well!