Is making minor adjustments to an online tutorial for a coding assignment considered plagiarism? For one of my pieces of coursework I have to make a game engine and I wasn't completely understanding what my lecturer was going on about. So I looked online and found a tutorial on how to make one and its been really useful and I understand better how it all works but my coursework is basically just the tutorial code.

I submit it in a couple of weeks so it may be a bit late to start thinking this but isn't it just plagiarism? I'm not copy and pasting the code but I'm writing it practically word for word. But on the other had there only so many ways you can use OpenGL and make it look different and do what you want it to do.
General Rule: Make it clear what your own contributions are, and where the other stuff is coming from.

Simply adding an explanation to your coursework on how you followed the tutorial, how far your modifications went, etc makes sure that you are not committing plagiarism. However, it is still perfectly plausible that your lecturer decides that the extent of your own contribution is insufficient. Thus, I would recommend checking in advance of the deadline.