Pants and related words

Hi, I was wondering about how words came about to be whatever they are now.

Specifically, the weird ones that pair weirdly in grammar.

Certain words (nouns) like pants, spectacles, sunglasses, scissors very obviously have a little twist as to how to use them in a sentence or phrase.

All of the above are generally addressed as themselves or with "a pair of" in front, but why does it have to be that way? I understand that with pants for example, the "s" in pants makes it a plural, which naturally gives the proper explanation for why it's used as "a pair".

However, there are no proper explanations for all of them.

A quick google search reveals that pants is defined plural because in olden days a singular pant is worn individually, one at a time on either side of one's legs.

Scissors are scissors because they are paired with 2 things, the left and right side of a pair of scissors. It on its own is still singular. This brings me to another burning question, which came first? The act of scissoring something or the noun of scissors? Extensive Google searches have proved futile, and I am cracking my brain over this. If the verb came first, and the act was to scissor something, it might still make sense to name it a plural, because to have that act of scissoring, you'd need 2 items or more to provide the shearing movement.

However it is still a very gray area as to why it is defined plural, and while we know many words pronounce comfortably in their plural, it's only because of habit and practice.

Pants are sometimes reported as a singular pant in many articles for clothing, such as "a floral pant is a summer necessity".

Jitters, are commonly known plurally, and never singularly, because a singular jitter never exists. It's a logical thought process, because you never get a single jitter, because the word jitters is used to describe the feeling and whatnot, rather than a singular action such as "shock" ("I got the shock of my life, and I still get the jitters thinking about what happened that fateful night").

Does anyone here have a sound or proper or proven reasoning why some words are defined plurally without it actually being plural?

Do enlighten me, kind souls