Questions about Purebreds

So I rescued a Great Dane and have had him for over 2 years now. He’s my first dog and I’m trying to understand the purebreds situation. I got his bloodwork done and he’s 99.9% Great Dane but this wouldn’t be recognized as purebred and only full bred which I don’t mind but never knew existed. Can someone explain how I can go about getting him registered and how that .1% makes a difference in anything. Ai has been somewhat helpful and explained that most all dogs will not be 100% purebred and it’s actually hard to keep them pure bred through the lineage but what else is the genetic differences between Full bred, Pure bred, And so forth. Whether you’re just very knowledgeable, own pure breeds, or just found articles please teach me what you know 🙂 If you could send links I could read that are legit I also wouldn’t mind that. Thank you for your help and I hope to learn a few things. 🐕 🍞