@nprofile1q... Really good questions.
I installed LineageOS myself. I was looking for a smaller phone, and started off thinking "this is ridiculous", but decided to give it a go after reading a Reddit step-by-step for installing LineageOS. If I couldn't replace Google on it I would still be running my huge SailfishOS phone.
I knew it would work in Aotearoa New Zealand by comparing the supported bands and protocols.
When 3G was turned off I had to update the firmware, which in itself was trivial,. But until 3G support actually stopped I couldn't be cerrtain I'd been successful as the network continued to tell me the phone wouldn't be supported. But it is, and it works well at home, in Australia and across Europe.