Caution: Spoilers Below!!!
 

There was a lot I didn't understand in this movie. What the hell is up with the CGI monkeys? The black guy said he was a hologram but he took food and ate it? THX was allowed to have sex with LUH after he was captured and put on trial for sex crimes?!?! What was with the huge mob of people outside the void prison? The robot cops were awfully pathetic, slow, and dim-witted. And what exactly did THX discover when he climbed out?
I liked the movie but I guess I like my sci-fi to be more meat-and-potatoes. this one had a lot of head-scratching...

The monkeys (and little people) lived outside of the protective shell around the city; they were mutants due to the radiation on the surface. They mention this in the prison scenes.

I think he was able to interact with physical things, but is only a program. Not the same as a Star Trek hologram.

With LUH and THX having sex again, I would guess that they found each other by accident in the prison and it just took a while for the authorities to find them. That or it was a dream, but I can't prove that.

It was just a large thoroughfare that they were trying to get across but the direction of the traffic wouldn't allow it.

Yes, they were.

I think what he found is sort of up to the viewer to decide. Did he find a real escape or just a world not worth living in?

That you may not like it. It's generally regarded as a great film that's not to everyone's liking. Some find it boring. Some find they don't understand it. That's what an abstract film is. It puts the issues out there and allows you to come to your own conclusion.

Well, I thought it was brilliant. Having also seen American Graffiti recently for the first time, I'm convinced that we lost out on the career of a potentially great director because of the success of Star Wars. Once the money started in, he started seeing green, and wasn't as involved with the Empire Strikes Back due to starting his own company/studio etc.

I wonder to what heights Lucas would have risen (artistically) had he not been sidetracked by the Star Wars juggernaut.

I'm not saying that his work with Star Wars after the first film wasn't meaningful - just pondering what might have been given the incredible trifecta of:

THX-1138
American Graffiti
Star Wars

Then nothing for 25 years. Then:

Star Wars: The Phantom Menace.