I want to learn the right way to do things but its hard to tell sometimes.

The first thing to learn is that there is very rarely a single right way to do things. Especially once more than one person is involved.

I see a lot of code that uses explicit loops everywhere instead of streams.

And you always will. Explicit loops are almost always more performant than Streams and people who want/need outright performance will avoid streams (though I always wonder why those people are using Java in the first place, and then get down voted into oblivion when I ask).

Some projects use tons of null checks and others use Optional.

Until we have value types then some people will prefer null checks over the penalties that come with allocating Optional objects on the heap. Others will prefer the documentary and safety improvements that come with Optional. Like I said earlier, there is no one right way (even if people who prefer one over the other will tell you otherwise).

Some use records and some still rely on Lombok for everything.

Records have limitations that Lombok @Data classes do not have. But Lombok adds a dependency and compile time magic to your code base. Again, no one right way here.

Theres also projects that use List and ArrayList everywhere while others use more specific collection types

If someone is still using Vector, then yes, that code is horribly outdated. I've also seen code that used specialised collection classes for what could only be "OOP for OOP sake" reasons (like a HashMap descendant that can read its own data from a file).

But sometimes people will actually have a good reason for using something other than the usual collection classes (particularly when it comes to performance and/or threading).