Build your own cloud storage with Syncthing
Cloud storage is often a type of file synchronization service (such as iCloud or OneDrive). While it can provide excellent value if you limit the synchronized files to a relatively low number (10-50GB), the price can quickly spiral out of control if you require a lot of capacity.
Luckily, you can build your own free cloud storage by using the storage capacity of your own devices with Syncthing.
Syncthing allows you to securely link your devices and synchronize all files within a given folder. The devices need to be simultaneously - but not continuously - online to synchronize. If you want the best possible experience, one device should ideally remain online at all times (for example, an old laptop).
Syncthing is not a back-up solution, but building a back-up solution on top can easily be done depending on the importance of your files and your technical expertise. For example, periodically copy-pasting your files to an external drive or USB-stick would be an accessible first step. If you can limit the truly important documents to less than 10-20GB, you can also periodically compress and encrypt those important files and use the free tier of a cloud storage provider as your offsite backup.