I’m using pCloud. Create a local folder on the desktop or laptop, for instance “D:/Zettelkasten”. Create a subfolder with the same name on pCloud Drive, but this has to be a subfolder in /pCloud Sync; so “/pcloud Sync/Zettelkasten”. Go to pCloud, click on the Sync icon and click on the Add New Sync button to sync the 2 folders. For me this works very well.
But if you delete something, it disappears both in the local and pCloud folder. So I also regularly make a backup by compressing the local Zettelkasten folder in a zip file and storing the zip file somewhere else. I keep the last three zip files, so when I make the fourth backup I delete the first.
To sync with my mobile or tablet (both Android), I’m using Foldersync (the Pro version, it’s about 5 € and you get rid of annoying adverts). It’s straightforward to use, just create two paired folders, the one on pCloud sync and a local folder. I only sync in one direction, from the cloud to the mobile or tablet. I find it easy and efficiënt to write notes on desktop or laptop, find references, embed figures or pdf’s, … and see relations in the graph view on a big screen. On tiny mobile screens this would cost me much more energy and I would lose my focus. If I want to write a quick fleeting note on mobile or tablet, I use Standard Notes. I convert those to permanent notes in Obsidian with proper backlinks and embeddings when I’m back behind the desktop.