If you only want to sync files between your devices, I would recommend taking a look at Syncthing. It syncs files without relying on a central server and is much faster, as it can sync within your local network and is therefore not restricted by the speed of your internet connection.
For sharing files with others, I use a self-hosted Nextcloud. I mounted my Syncthing folder as external storage.
If you use GNOME as your desktop, you can also log into some online services in system settings and then, for example, access Google drive via the file manager to upload or download files.
I believe there are also third-party tools for some cloud storage providers, but I have no experience with them and therefore can’t make any recommendations.
Some cloud storage providers probably don’t see Linux as a large enough market to justify the development of official clients. Some, e. g. Microsoft, could have an interest against one to not make alternatives to their own products, e. g. Windows, more attractive than they already are.