Its hard but rewarding, and perfectly doable given you want to do it.

Note that there are a few mandatory requirements for a port to be feasible, like unlocked bootloader and vendor sources available for kernel.

As newby, you also want already existing ports to start from.

Also, some powerful hardware to build on (32+gb ram, 10+ cores, 100+GB storage).

Also older (up to A11) can be real bitches, after A12 things are improving a bit.

You need to copy or grab or create the device tree files. Usually by cloning a similar device. Vendor files are extracted from stock rom. Kernels, you need an official release from vendor to start width.

It's a sub-brand Xiaomi phone from a couple of years ago. Going strong... From A10 to A13 and next year A14 too...