iMac usb ports acting really weird?

Hi there,

So I recently got this iMac late 2015, someone bought it and it came locked, and they purchased a new logic board for the iMac but never fitted it before selling it off, and so once I put the new one in and tried to install macOS onto it, it wouldn’t register any of my keyboard inputs, nor a usb stick attached to it. I went down a multiple day rabbit hole of trying many different things, I tried 5 different keyboards, including going out and buying a very basic 5 dollar office keyboard, as I heard that if the keyboard is recognised as a composite device instead of a basic hid device it wouldn’t initialise fast enough with the macOS bios to register inputs to boot into recovery mode. This ended in me buying a raspberry pi pico in order to make a super basic keyboard press emulator, I was at my wits end. My main pc broke before I could finish that, but that is irrelevant. I got bored one night and wanted to see what would happen if I put an ssd with windows on it into the iMac, and I was honestly surprised it booted without issue, and I figured that once windows os fully loaded it would load all of its drivers for recognising usb devices, however none of my keyboards still work. If I plug in my mouse that has rgb, the rgb will light up for a brief second before turning back off, however if I’m wiggling the mouse as I plug it in it still won’t detect. I tried all my usb sticks as well, and none of them would recognise. I have verified that it delivers power through the usb ports, as a usb powered light works, and it can charge devices, so I thought it would be a case of an issue somewhere along the data lines, or with the usb controller, I’ve learnt there are potentially two that handle half of the ports individually. However, randomly when I plugged my phone into the iMac via usb c, it detected as a storage device in windows, and opened the file explorer. I then tried with a usb c ssd enclosure, and sure enough that was detected immediately as well. I did the same with a regular usb external hard drive, and this also worked. However these are the only devices that seem to work for transmitting data and being recognised.

I’ve been having a lot of fun with the weird challenge this iMac is giving me, however honestly I’m just a bit stumped about the entire thing. Is there anybody who might know what the issue is, or things I could check to find out the problem here? And help would be very greatly appreciated!