I run a Thinkpad with NixOS and KDE, a Pixel 9 with GrapheneOS, and an Amazfit watch paired with GadgetBridge on my phone.
It's a testament to the hard work of the FOSS maintainers of these projects, and the spirit of open source, that everything works flawlessly together without any cloud service sucking up my data. For example, I can control youtube and music playback on my laptop with my watch because KDE Connect syncs my laptop and my phone, and gadgetbridge syncs the phone and the watch. The breezy weather app on my phone can automatically push its data to gadgetbridge which in turn pushes the data to the watch. And so on. So many little things, developed independently, working like a single well oiled machine.