You can re-use parts of existing code and libraries which makes little sense to rewrite from scratch. If the source code you re-use are of substantial size and scope then make sure to write about it on the itch.io page for the game you're submitting. An example is if you re-use a character controller, or a complete save game system, or an algorithm to generate procedural dungeons. I know the line in the sand can be hard to see but if you are in doubt then you're better off being honest and write about it.