Nextcloud

Nextcloud is a self-hosted platform that puts file storage, calendars, contacts and collaboration under your own roof. You get sync clients for desktop and mobile, WebDAV and CalDAV and CardDAV support, plus optional apps for mail, group chat, video calls, tasks, notes and photo management, and an in-browser office suite.

Families use it to replace a consumer cloud account, and companies use it for shared team folders and document editing.

Running it on your infrastructure means the data stays where you decide, there is no per-user cloud bill, and a file copy plus database dump restores the whole setup.