Most often the culprit is PSU or RAM.
Did you put the RAM in the right slots? Mobo manual will tell you where. Do you have 2 sticks or more and were they sold as a kit? Because in general you can't mix and match. If you bought a kit of 2 sticks, one of them can be bad.
Use something like Memtest, Prime95 etc to test them, one stick at a time.
For the longest time I could not figure out why my PC rebooted randomly, turns out it was one stick out of a 2-stick kit that was bad. It's been over 10 years since but that was not easy to hunt down. Because it was so rare. Took me a year.
I don't know anything about your PSU but it should be fine. The last thing I can think of is CPU overheating. Is your cooler fitted on right? Did you use the recommended amount of paste? A grain of rice. The paste should only be on the CPU, not in the socket or anywhere else nearby. That would be bad.
The CPU cooler shouldn't be clamped down too hard, leads to unexpected things. You shouldn't be able to rotate it with your hand. That is tight enough. If I recall correctly, if it is clamped down too hard, you can loose memory channels etc.
Generally, if a GPU has issues, you get blackscreens. Computer still on but no output. So I doubt it's that.