Nextcloud

Nextcloud is a cloud-based "file sync and share software". It offers

syncing of OS-based directories (Linux, Windows, macOS) with the Nextcloud server ("Desktop Sync client")
a browser-based desktop (the "Web interface") with various "apps" (including "groupware").

apps

Files: the (synchronized) files on the server
- files can have tags and can be shared
Photos: organization of images
Activities: a log of changes that happened in the apps -- sometimes links are missing to changes, it is not possible to directly jump to them
Mail
Contacts
Calendars
Deck: Allows note-taking, hierarchical organization into deck -> board -> stack -> card
- Card: has users, description, attachments, timeline, tags
viewers for videos and images
project: collection of items (files, chats, etc.)