No, you can figure out a miniscule portion of it which may be enough (How do I know this?)… Even Epic developers will only work on a specific portion of the engine and will specialise in certain areas, if it was a matter of skill and budget then the engine would only be any use to a AAA company… No one man band or small team, or even mid sized teams will ever understand how it works end to end in a reasonable timeframe and it’s not really a matter of skill… Because if you worked in a team of 75 with a bunch of dedicated engine dev’s, you’d build your own instead of trying to constantly reverse engineer what someone else has done. Again in near enough every scenario it’s impractical if not improbable to rip the engine to pieces…
Luckily enough UE is quite indie friendly.!
Small teams can stack their own stuff sure, they might even be able to add modules and expand the engine like again you can do in Unity via plugins. But game development != engine development they are two completely separate things although they share common ground.