Am I going to delete my source files?

Hello,

 

Please consider the following scenario in Lightroom Classic.

 

1-I select some photos in lightroom classic and mark them for synchronisation with the cloud

2-They show up in my web-based Lightroom.

3-I delete them in my web-based Lightroom.

By @kasra_yousefi

No. Deleting synced images from the cloud will delete them from all the connected cloud apps, but does NOT delete them from Lightroom Classic. In LrC the deleted files are simply "unsynced", so will disappear from the All Synced Photographs special collection, and from any synced collections they may have been in, but they remain in the catalog in the original folders and any unsynced collections they may have been in.

You could try it on one un-important photo and find out yourself (and then let us all know)