Center control

It's not useful to think in terms of central control, or at least, it's more useful to think in terms of active pieces.

Active pieces are some combination of mobility (influencing many empty squares) and being in contact with weak points (typically weak pawns or squares around a king).

People are told to control the center because it's a typical way to have active pieces. In some openings you let your opponent establish a nice center, and then you counter attack it early in the middlegame. In that case ideally your pieces are active because they're in contact with weak pawns.

Sure, the center is important, and for many players, they will improve their results by challenging themselves to follow the principles more closely.

But it's tricky to give an exact number for the value of the center because there are always counter examples in chess.

The center is completely overrated the periphery is where it's at attacking on the flanks you never going to find anything new in the center it's like a crowded Subway

One way of understanding the importance of the center is to consider the following position while thinking about the difference between what the squares behind the pawns represent vs the squares in front of the pawns.

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Why might it be more useful to have a mass of central space (white) vs two masses of flank space (black)?

Think about this answer in the context of what you know about piece activity and piece value in general. For example the reason a rook is worth more than a knight, or why a knight in a corner square might be worth less than the same knight not in the corner.

#3
The center is of central importance. Capablanca said that controlling 3 of the 4 central squares is enough to win a game.
space advantage is overrated
pawn formation is very important: pawns are the soul of chess. If you place a piece on the wrong square, you usually can rerout it to the right square later, but you cannot undo a pawn move, they cannot go back
a passed pawn is a potential queen and thus very powerful
"If the opponent controls the center it is difficult to do anything." That is right. Though control does not necessarily mean occupation. You can also control the center from a distance. a bishop on g2 controls the central squares e4 and d5 from a distance.
More important than center control is king safety.

People measure aspects of a position differently and it can be subjective at times also.

One (of I'm sure many) ways is to count the 4 center squares "controlled" by pawns or pieces of each side (pawns winning control over pieces for reasons already addressed). Then, you may have squares that are considered "contested" by an equal number of pawns( or pieces of the same value) of each side.

Some methods include the central squares of files c & f. Some methods distinguish between kingside/center/queenside. Some methods have it "baked into" space and/or pawn structure.

Larry Evans method-
https://www.chess.com/forum/view/general/gm-larry-evans-method-of-static-analysis

Dan Heisman method-
At one time included space with Pawn Structure (Novice Nook April 2003). You might find this link very useful...

https://web.archive.org/web/20140625052220/http://www.chesscafe.com/archives/archives.htm#Novice%20Nook

Jeremy Silman method-
Space (k-s/center/q-s) and the imbalances.
Special attention to the comments at move 18
https://www.chess.com/article/view/the-pawn-center-strength-or-weakness

Intermediate Course-
Center Control (individually)
Lessons 21-24, 30-38
https://chessfox.com/free-chess-course-chessfox-com/