So, I have my pc and laptop synced with just a single file but they don’t sync unless both devices are online at the same time. I thought it should update the files when one of them is on am I wrong?
No. How would it do that?
No, that is not how it works. Syncthing only synchs between your devices. No “cloud” or anything has your files.
As others have stated there is no central storage. So files only can transfer among two devices that are connected to each other and powered on at the same time.
if you need a transfer solution you either need to have an always on central server that you maintain also with syncthing running on it or use one of the cloud storage providers like Google Drive, OneDrive, Dropbox, etc…