They are fastened to the top of the trousers at the front and back

[GoldfishLord]

braces
(British English)
(North American English suspenders)
[plural] long narrow pieces of cloth, leather, etc. for holding trousers up. They are fastened to the top of the trousers at the front and back and passed over the shoulders.

Source: https://www.oxfordlearnersdictionaries.com/definition/english/brace_1?q=Brace

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Does "at the front and back" modify "fastened" or "the top of the trousers"?

 

[Barque]

"At the front and back" refers to the front and back of the top (the waistband) of the trousers, and it qualifies the entire phrase, "fastened to the top of the trousers".

The braces are fastened to the trousers.

Which part of the trousers are they fastened to?
The top.

Where exactly at the top are they fastened?
At both the front and the back.

Try and understand the sentence as a whole and it'll be easier to figure out how each component works.

I assume you know what braces look like. Picture them in your mind, or google for an image and then look at how they're fastened.

 

[GoldfishLord]

It seems to me that "at the front and back" modifies adjectivally "the top of the trousers".
What do you think about that?

 

[Barque]

Possibly. I understand the sentence and the grammar, so I'm not fussed about what the exact term for it is.

 

[Tarheel]

I would probably say they are attached to the trousers (pants). Why? I can use fewer words that way, and they would have to be attached at the top. Therefore, "at the top" is unnecessary. (Where else could they be attached? They hold your pants up.)

@GoldfishLord What do you think? Do you think my way is less clear?