Flatpaks are a convenient way to install software on Linux. Flatkpak is popular because its convenient to package applications once, work across a variety of Linux distributions, and is isolated from other applications.
There are so many cloud services, its practically raining them. Bah duh. Awful dad joke aside, there are countless cloud services that allow for seamless syncing from device to device. What is lacking, however, is Linux compatible services. Most popular services like Google Drive, OneDrive, or iCloud only work with Windows and MacOS. Being the FOSS friendly folks that we are, we wouldn’t really want to rely on those anyway.
Every service on this list offers a native Linux client. Not some work around. Not a paid third party application. Only legit Linux compatible programs.