Joplin is a free, open-source note taking and to-do application, which can handle a large number of notes organized into notebooks. The notes are searchable, can be copied, tagged, and modified either from the applications directly or from your own text editor. The notes are in Markdown format.
The notes can be synchronized with various cloud services including Nextcloud, Dropbox, OneDrive, WebDAV, or the file system (for example with a network directory). When synchronizing the notes, notebooks, tags, and other metadata are saved to plain text files that can be easily inspected, backed up, and moved around.
The application is available for Windows, Linux, macOS, Android, and iOS (the terminal app also works on FreeBSD).