Hi Anna,
In addition to Christopher's answer, there are other major reasons not to contemplate replacing just the logic board.
Prior to the iPhone X, the only component you needed besides the motherboard was the home button, as it was paired to the logic board and could only be replaced by Apple. When the X came along, that all changed with the introduction of Face ID; now you need the front camera assembly, the earpiece speaker assembly and the logic board.
Since then it's only gotten worse; on your phone you then need the battery and the screen in addition to the other parts I've mentioned. So now if you want everything to work you have to replace most of the internal and external components of the phone in addition to the logic board.
So due to all this repair-antagonistic parts pairing, it's to the point where you're much better off listening to @cwi's advice and just replacing the whole phone rather than trying to replace the logic board.
On the other hand, it may well be worth your time to find a repair facility that has expertise in board-level repairs and get a diagnosis from them. At that point you'll know what you're dealing with and can evaluate your options. If you do get a diagnosis, you're welcome to come back and share it with us and we'll be glad to offer suggestions.