1. About UEFI:
In Mac, it's called EFI, not UEFI.
The files are stored in a hidden partition on the HDD/SSD name "EFI". If you can unhide it, you can open the files, study it, make changes. This is where people like Dosdude1 worked to create their patchers to install newer MacOS on unsupported Macs.
On an USB instaler, there is also a hidden EFI partition, which by default can only be seen by the iMac SMC+NVRAM (BIOS equivalence in PC).
2. About Windows on Mac machines:
You can create a WindowsToGo disk (still remember how to do, don't you?), plug it in the MBP. You will have a working Windows 10 64bit machine.
It will lack some drivers, which can be installed using BootCamp 6, and a few drivers require BootCamp 4.
BootCamp 4 USB can be created using the BootCamp Assistant in your current Mac.
BootCamp 6 can be downloaded somewhere on the Internet. Google for it.
And Windows guy, can you tell me what kind of fancy upgrades you have done on a 13" Windows Laptop, with solderd CPUs? Not very many, I guess. The same will applied to the MBP 13" you are holding.
I just laid my hand on my first Macs several months ago. I would stay away from a laptop thing if dreaming of upgrades.
Find some old Mac Pro or iMacs, and you will get your PC expertise ramming.
Yeah, I thought for a moment about installing W10, but since it's an Apple product, the customer likes Apple and expects Apple, even though I'm sure I can configure W10 to make him love it as much or more than his macOS, and I read about possible problems with drivers, WiFi or whatever, I decided to install Catalina instead. Came out fine.And to my experience, installing Catalina to a basic model 2009 MBP or iMac or MacPro is just as easy as installing Windows 10 on a Core 2 Duo PC/Laptop.
After creating an USB installer, you just need to boot it up and let the machine do it jobs, click mouse here and there. Things done. There are actually nothings to write home about.
The hard part is to create a proper USB installer, which dosdude1 has done for us.
I don't know why the tone with the "fancy" updates, I indeed wanted to swap the CPU but then I saw it was soldered, so it was not worth the effort. The RAM was horrible, not matching sticks, then the HDD, some ports were not working correctly... I made his MBP so much better and fixed everything. Fantastic job.
I don't know if you read correctly, but this is not my laptop, and the customer did not buy it to upgrade it in the future. I simply saw the opportunity to upgrade it and make it so much better, so I did. Not sure why you recommend me now to buy iMac if it has nothing to do with the topic.
Installing Cataline definitely is harder than installing W10. First, it's not "official", so you have to spend time reading all the extra stuff besides the official way of updating macOS. Second, your average Apple/Windows customer barely knows what is an OS, let alone create a bootable USB, or what in heavens disk format means, or file system. For us experts that have been doing this for many years, it's easy peasy, but for others that have used all their life the PC/Mac just as tools to work, it's something that they completely ignore. So, we exist, to do the job. They will be good at something that we aren't. Definitely it's something to write the customer about, because I can put this very example, my customer does not know anything about all this stuff, and he's so excited because of everything I fixed, installed and upgraded.
For a mechanical engineer it's easy peasy to calculate the stress-strain analysis for a given structure and "it's nothing to write home about". Guess what? Companies pay them hundreds of dollars because they don't understand anything about engineering, so it's a very complicated issue they don't understand at all.