Why is my phone restarting by itself it has a custom os

Your phone was bootloader unlocked and runs a custom ROM.

Which carrier is it subsidized by? A and V (AT&T/Verizon) phones are tightly locked down and will refuse to start with a "mismatched" ROM; they do this by using KNOX and enabling Boot Signature Enforcement. Sprint (P) phones are just as bad. YES, even a US Samsung ROM from the Unlocked series does not run on these!

That said, they usually don’t bootloop. It tends to be a message about an incompatible ROM, which does not clear up without reflashing it back to the original factory ROM it shipped with (though this could be their way of doing it if they don't want to come out and say it).

The Exynos U variants can be bootloader unlocked, but not the US QCA phones since they use the same boards but change the ROM signature check and without a carrier lock when purchased as a "U1" phone; these can never be locked. However, this will trigger the eFuse and kill the Samsung Health features (HIPAA security issues) and KNOX secure workspace.
(Yes, they could have easily just killed the parts that need the HIPAA security guarantee, but Samsung kills the entire app)