Take a look at the Game Modes override functions.
Specifically Choose Player Start and Find Player Start
In GameModeBase.cpp :
AGameModeBase::FindPlayerStart_Implementation
AGameModeBase::ChoosePlayerStart_Implementation
UE on Github :: https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Source/Runtime/Engine/Private/GameModeBase.cpp
For those seeing a 404 error on GitHub for the link provided.
You need to have your Epic Games account linked to GitHub. Here’s the official walkthrough.
https://www.unrealengine.com/en-US/ue-on-github
Bonus link to the UE Source “File Finder”
https://github.com/EpicGames/UnrealEngine/find/release