Restic is a modern backup program that can back up your files:

from Linux, BSD, Mac and Windows

to many different storage types, including self-hosted and online services

easily, being a single executable that you can run without a server or complex setup

effectively, only transferring the parts that actually changed in the files you back up

securely, by careful use of cryptography in every part of the process

verifiably, enabling you to make sure that your files can be restored when needed

freely - restic is entirely free to use and completely open source