How do you break down the doors that are in the background?

[3D Master]

See subject.

[Fel]

"See subject"...

There are 4 small-ish doors with an unlit rune above them in each zone, they are openned by playing on higher boss cell difficulty (acquired by beating the hand of king in each difficulty).

Other than that, there is a big door near the start that doesn't do anything (as far as I know), and a few "cell" doors in the story sections.

[3D Master]

Four smallish doors? There's loads of doors in each level, you walk buy them and it says "open door" with the interac button, the character then bashes on the door, and that's it. Similarly the cell doors with a prisoner in them, either with water or without, and some dialogue shows if you interact with those. I've seen videos where the character breaks down those doors.

[Fel]

Those are the "small-ish" doors.
Slightly bigger than your character wooden doors, with a round top, with a black circle in the middle and a dark circle above.

The circle above lits up when you can open it (1 door for each active boss cell per zone, up to 4).

Basically, beat the Hand of king in the normal difficulty, then activate the cell in the long tube at the start for the first door per zone, then beat it in 1-cell, 2-cell and 3-cell difficulties to get all 4 boss cells.

It is more or less a compensation mechanism for playing at a harder difficulty, and the videos you have seen where people open them should also have a red cell and a number in the bottom right corner of the screen to indicate how many active boss cell that person is playing with.