Which way are you trying to go? And what exact versions of Lightroom and Lightroom Classic are you using? If you are referring to synchronizing from Lightroom Classic to Lightroom (for the cloud) then here is an excellent tutorial for you to watch. It runs for about 17 min. Take the time to watch it because I believe it will give you a good foundation for what you are wanting to do:
Someone else might correct me. But "I think" all you need to do is be sure to be signed in to Lightroom Classic with the same Adobe ID and password and enable sync in Classic. On the "Lightroom Sync" tab in preferences, be sure to designate where you want your synchronized images to be stored, and the synchronization process should take place. I know whenever I add images to Lightroom Mobile (i.e. take photos with the Lightroom camera on my iPhone) they are automatically downloaded to my main computer that has Lightroom Classic.