I have some comments but I am not here to drill you - just hoping to assist. First - the 24pin only is not a test you need to also plug in EPS (CPU) power. 24pin only will not work. When you swapped your PSU into friends PC - did you use your same cables from your PSU (24pin and EPS) ? You want to rule out a bad cable.
At this point (its some work but needs to be done) you’ll want to remove possibilities that there are no grounding or shorting issues and this will involve removing the guts from your build and doing some bench testing using the minimal set of components needed to verify CPU/Motherboard/PSU at first. Then slowly working your way back up.
Put the board on the box it came in to use it as a make-shift test bench.
Socket the CPU and cooler plugging a fan into the CPU Fan Header. For this test use the stock cooler if your CPU came with one - Double check that CPU is oriented properly. Line up the little triangle on the chip with the one on the socket. The Chip should just fall into place without and pressure applied. Close the lever.
Install your Ram - make sure its properly seated.
Install a GPU - make sure its properly seated.
Make sure the rocker switch on your PSU is in the off position
Plug in the 24pin from PSU to the board make sure its fully seated - both PSU side and Motherboard.
Plug in a EPS 8 PIN (4x4) to one of the EPS headers. (Only need to use one) Make sure cable is fully seated in both PSU and motherboard
Plug in PCIe power 6 or 8 pins to the GPU depending on your cards requirements.
Connect graphics card to a display
Turn on display
Power on your PSU
Press the Power button on the motherboard
IF everything is alright she should boot to a post screen. Your CPU fan should be spinning, The lights on the IO Shield will light as well as the debug readout and diagnostic LEDs and LEDs for the bios (indicating which bios is active)
This should help to verify that CPU/Motherboard are good. - If it doesn’t still - then I would actually pull the CPU from socket check for any bent pins etc and then re-seat it , remount cooler and retest. (may just want to do this step first actually as step 1 above)
If it still doesn’t power/on post I would still try using a different PSU / cables just to be sure - IF same result then I’d probably try the same setup using a different motherboard.
IF it does Post - then you will want to start adding the rest of your components back to the setup (still outside of the chassis) one at a time (powering off completely, add component, power on re-test) until you have all components back in the test setup as you used in your chassis, and system is powering on running as expected.
Before adding things back into the chassis you want to double check that your motherboard standoffs are positioned correctly - that none of the wires for fans etc have obvious signs where they may have been pinched etc etc. Just the common sense stuff.
You could just try replacing the motherboard first but you pretty much have to tear down your system anyway to do that so I feel its worth the time/effort. Especially if you go through all that (replace mobo) and you end-up with same issue -