At some point when I downloaded Lightroom Classic (previously only using Mobile Lightroom), all my presets from Lightroom Mobile synced to my Lightroom Classic. I quite literally have no idea how I made this happen, but I accidentally deleted all my imported presets in my Lightroom Classic, and now I can't figure out how to get them back. I can find lots of information about getting presets from Lightroom CC to Lightroom Mobile, but not the other way around.
Presets do not sync from Lightroom mobile to Lightroom Classic. What you should do is apply a preset in Lightroom mobile, wait till the edits are synced with Lightroom Classic, and then in Lightroom Classic create a new preset from these edit settings. Maybe that is what you did in the past. Another option is to install Lightroom desktop if you didn't do that already. Lightroom desktop can export presets that you can then import in Lightroom Classic.
Oops, I think I didn't explain properly. When I downloaded Lightroom on my computer, the mobile version, it included all my mobile presets.
I then downloaded lightroom CC on the same computer, and at some point my "User Preset" folder absolutely showed up on my Lightroom CC because I had absolutely no idea how to save my presets on my CC version lol. They just appeared somehow.
So what version of Lightroom are we talking about? 'Lightroom CC' does not exist anymore. It is now simply called Lightroom. That version and Lightroom mobile share everything. They use the same cloud as storage and as catalog. That means they also share presets. Lightroom Classic does not share presets with these two, so what I wrote still stands.