@franco_vazza These "local snapshots" are made on your main disk and happen independently of when your back disk is plugged in. See here:

https://support.apple.com/guide/mac-help/about-time-machine-local-snapshots-mh35933/mac

I have my backup option to "hourly" but only plug my SSD disk in once a week, it then it automatically does the hourly backup - but only when it is plugged in.

I'm not sure if setting it to 'manual' would also affect the snapshots. You can start timemachine from the finder without your backup disk and see if the backups are there. If the disk is plugged in too, you will see a slight difference in color on the "lines".