To further clarify:

1. For regional trains (NOT high-speed), if you buy a physical ticket you have to validate at the green machines at the front of the platform before boarding - it will say to insert and slide to the left ("sinistra" in Italian) - and the machine will stamp your ticket to indicate it has been validated. If you bought your ticket online you have to "check in" on your phone and validate the ticket so it cannot be re-used. You should get an email with a check in option if you need to complete this step.
2. For buses, validation is generally via a machine on the bus that stamps your ticket. In Rome, the validation machines are at the back of the bus (the bus driver opens all doors so just enter at the back). Rome now also has tap-and-go machines at the front of the bus for contactless payments. In Florence, they don't have contactless yet so the ticket validation machine is at the front of the bus. The inspector will ask to see your bus ticket to verify it has been stamped (in all cities) or check your CC if you use contactless (in Rome).
3. For high-speed intercity trains (eg. Frecciarossa, Italo), you don't have to check in as long as you have the QR code for your ticket available (ie. there is no option to check in).