Can I upgrade my i3 8100 to a i9 9900t?

"You are limited to what Apple used for CPU’s. Your system is limited to these iMac19,2

The highest performing CPU your system can support is 3.2 GHz Core i7 (I7-8700) CPU

Between the limits in the systems firmware as well as the systems cooling and its power it will not support the 4.40 GHz Core i9 (I9 9900T) CPU"

If you want upgrade to i9 9900/9900T/9900K/9900KF, you must buy iMac i5 8600 because iMac i5 8600 has tray to install Nvme. It's better for your iMac. Check out this video: https://www.youtube.com/watch?v=pAdYWkXJ...

"uh, nope you cant

HOWEVER in my theory you can get an M1 Mac Logic Board and solder out M1 chip so more power on it

WARNİNG:NOT A TESTED THEORY"