I wish people who say this would give specifics. I cannot imagine what is confusing you. I never used Blender before using 2.79 for a few days to do the donut tutorial and then spent a few months using both 2.79 and 2.80 equally and then spent several months using 2.80 but following along with 2.79 (and earlier) tutorials before the massive explosion in 2.80 content on youtube.
From my beginner/amateur point of view, 2.80 is just a SLIGHTLY rearranged (for the better) version of 2.79 that lacks the Blender Internal render engine and has a different fluid simulation engine with slightly different settings and very different results, also slightly different keyboard shortcuts outside of the ones you press over 80% of the time, which have not changed.
I have also the same problem of having some files that work on 2.7 and other newer created with 2.8.
For this reason I just rename the old files with 27 at postfix so I know how to open them.
Disclaimer: I seldom version my projects with more than 10 versions, so there will never be a case with a file named project26.blend created in Blender 2.8 and project27.blend to get me confused and open the last with Blender 2.7.
Believe it or not, I recently finished a project where we animated a 15-second intro for a live stream and made it look like it was an 80’s or 90’s news graphic intro. Instead of forcing a newer engine like Cycles or EEVEE to render like an old render engine does, we just did the whole project in Blender 2.49.
Oof. Guys. It. was. rough.
It was kind of nostalgic to see a side-by-side comparison with how far Blender has come and what it was like to use Blender when I was getting started, but holy cow have we come a long way.
I had a client who was using 2.49b as late as 2017.
I had to export or append backwards. And do all of the final set up in 2.49b
Even though I learned Blender with 2.49b it was wild to see how far we had already come. But also the similarities.
I think to the OP, I have empathy. It really takes a lot of effort to keep up to date. I was late adopting 2.8 just because I did not have the time to explore it.
But I did do it eventually.
Once I heard it expressed that software is a moving target. I think that describes it well.
You can’t stay idle for long. If you do you will get lost.
Good question - it would have made it a little easier because I could have just used BI in 2.7. We really wanted that clunky 80’s graphics feel, so we purposely limited our tool set to get a more “authentic” feel. It was one of those “do it for the art” decisions that we made with the client, even though we knew it would create some minor headaches.
In fact, after 2.49, “improving rendering” was equivalent to improving Blender Internal, just during 2.5 series. That means the features needed for Sintel (volume rendering for fire, faster BVH trees, bump mapping, color management, deep shadows …).
Cycles was added in 2.61.
So, 2.6x and 2.7x series stuff were about generic things usable elsewhere ( baking, viewport OpenGL render improvements, compositing, normals editing, multiview ) and bugfixing.
The last big feature related to Blender Internal was Freestyle in 2.67, during this Cycles rising period.