PC won't start - starts - won't start

Several years ago I built a PC for my wife in an old Dell case. I used Vandal switches at first because Dell has weird switches, but had issues. I would press the power button and the PC would power on for literally a second and power down. I sent the MB back to ASUS but the replacement did the same thing. I bought a new MB, same thing. I salvaged a HDD panel with power and reset buttons from another trashed PC and the new PC started. My wife needed a new PC while this was going on so I got her a Dell, I7, 8GB.

Time marches on and the Dell will not properly recognize newer BT earbuds. It only sees the mic but not the audio. Once the thing started with a different start button I bought a Cooler Master case and all worked just fine but I never used the thing. I dragged the PC I built many years ago and started working on it again. I had upgraded to Windows 10 a couple of years ago and it took a day for the PC to complete all the current MS patches. It's so old the MB does not have WiFi or BT but essentially it's new and everything works. I had installed a WiFi card and put in a BT dongle and everything worked just fine.

I finished installing all the software and moving everything she needs to the new PC. Not once did I have an issue restarting it many times. I moved the PC to our office and connected it but when I powered it on the old problem reappeared and it would only light up for a second. I dragged the PC back to where I worked on it but it still would not start. I dug up an old Acer power supply and replaced the still new Corsair TX 650 I had purchased when I first built the PC. The Acer power supply does not have the PCIe 8 pin connector but when I powered the PC on it powered up normally.

Based on this I have to assume it's the power supply that is the issue but I still can't connect the PCIe. I ordered a new power supply but, finally, my question is, can anyone explain to me the initial power on sequence from when the power button is first pushed? Does the power supply keep the feed up or is it something on the MB? If this does not work I will build another PC for my wife as all I need is a new MB, processor, and memory. Thanks