Why is my phone power cycling

That’s called “Boot loop”. Occurs usually when bootloader is broken by OTA update (that happens), flashing done wrong via USB or unexpected loss of power.

You can try to restore it via flashboot mode and ADB.

I guess, you tried to Install TWRP and forgot to install Magisk after flashing TWRP or something else?

I have been noticing this issue with my Z981 for the last few months, OTA update isn’t the issue as I never had it activated after I got it from a family member back in December 2018 and I’m sure that installing TWRP and Magisk isn’t easy or possible on this phone

And it does boot loop like once or twice a month as well