How do I sync LR mobile presets to LR for my Mac?

I'm not sure where presets are stored when you use the free mobile app without any Adobe plan, but I assume they are stored locally in that case. They might not sync if you then start a subscription plan.

It's the same process. If you create a preset on one of the cloud apps it will sync to the cloud and thus down into the other apps. So if you start the Lightroom desktop using the same login credentials as you are using in Lr Mobile, you should find all those presets will appear in the Desktop's Presets list.