Moto XT1921-8 code
 

Hi,phone with "someone else's" sim card asks for a code?
Motorola e5 go.
imei 359543096084450

How do I find out the code?

Depend on what code phone ask got few solutions:
1. If ask about network code need to unlock by code from the network operator.
2. If ask about the phone code you must ask the owner about it or can reset the device to the factory state but will be lost everything in the device.
3. After reset to factory state phone can ask about the google account to bypass it need use the manual for your device.
4. Other code who knows for what.

asks for the code when I put the SIM card to another operator

Mean network lock then need to buy from operator this code.

Unlock verizon sim card?