I need some help with understanding this particular variation

This is the Berlin defense of the Ruy Lopez, this is the L'Hermet variation.

The issue I have here is that according to all the main variations, the 5th response for black always lets white off the hook and gives white the opportunity to regain the original pawn that was taken.

After white goes 5.d4 can't black simply go 5...exd4 and take the pawn? This would surely prevent white from being able to regain the loss of a pawn, unless I am mistaken. If I am wrong, show me either how white is still able to regain the pawn, or at least how taking the pawn would lead to either a positional breakdown for black or a trap.

5...exd 6.Re1 d5 7.Nxd4 with dual threats of capturing on c6 and winning a piece with f3.

5...exd 6.Re1 f5 7.Nxd4 Nxd4 8.Qxd4 I just assume white is much better since black is far behind in development and isn't even close to castling.

There is an openings database you can try and use to learn also

https://www.chess.com/openings

What do you mean by winning a piece with f3? I don't know what you are referring to.

After Re1, the knight is pinned to the king. When the Knight on f3 moves, you are threatening f3, attacking the pinned knight with a pawn, therefore winning the knight.

exd4 is playable, but certainly risky if you can’t deal with the complications 

After 7.Nxd4 there is no knight on f3. What you are saying is that the correct choice is to attack the knight by using the pawn on the f-file by going pawn to f3?

It depends on what black does next, but generally, that’s the plan if black doesn’t respond to the threat.

Then don't worry about it. This isn't a useful question for you to be asking. Learn the basics of chess and worry about this stuff later.

https://www.chess.com/lessons

Specifically, this lesson answers your question.

https://www.chess.com/lessons/capturing-pieces/piling-up

Capturing the pawn on d4 allows the types of ideas showed in the lesson.

How can black respond to the threat?

Instead answering a bunch questions, I will leave a board analysis with moves and responses I think are plausible to reach.

I've gone through it and have completed the challenges within the lesson, although I would definitely like to be able to figure out how to apply it over the board, which is something I may need advice on.