Interfacing markdown:

• Joplin (pretty popular, haven't used)
• Standard Notes (same as above)
• Markor (feature-packed, a bit outdated)
• BeautyXT (maybe a bit too minimal almost, but gets the job done if you just want text based editing)

Interfacing orgmode:

• Orgzly (beautiful simple app that uses a superior syntax when compared to Markdown due to its built-in agenda and to-do structure)

Syncing flat files (might not be compatible with all of the above)

• Their own hosted server if available (easy but not private, some provide E2EE)
• Syncthing (easy and private)
• WebDav or an SFTP server (hard if self-hosted or otherwise easy through some providers)
• Termux + a Git server (pain)