If your iPhone did not encounter liquid or a hard floor, you'd have to start by replacing the back camera and see if it solves your issue.
This guide has the needed instructions, tools, and part.
1/ Skip steps 17 and 18. You don't need to remove the battery; whereas taking it out puts you at some risk of damaging your logic board;
2/ Step 21 is important. Disconnect those "interconnect cables" from the side depicted in the guide, not the other side, as you risk damaging many critical parts on the board;
3/ Tools and prying: don't pry at the board by inserting tools between the battery and the board. Don't use any metal tools inside the phone except the screw driver ONLY to remove screws.
I usually remove all the screws, disconnect all the flat cables that have to be so, then "lightly" hold the board by the SIM slot and pull it out. If I feel any resistance, I stop and look again for anything that may be forgotten, stuck, or at risk of breaking/ripping.
This is a simple repair that can easily go wrong, so tread lightly and don't force it.