it's not nearly as clear cut as that unfortunately. take this youtube link for example:
https://www.youtube.com/watch?v=dQw4w9WgXcQ
getting rid of the portion after the question mark just leaves you with "https://www.youtube.com/watch", which is entirely unhelpful to youtube trying to figure out what video you are even trying to watch. but then, sometimes it is safe to remove everything after the question mark, like in the link taken from the second image:
https://www.youtu.be/dQw4w9WgXcQ?si=Aa1Uc_fRHXC0ay85
where in this case, removing all of that gets rid of the source identifier but keeps the video. there's a few easy tricks to tell them apart that work for specific websites (for example if your youtube link looks like "youtu.be" instead of "youtube.com", everything after the question mark is for tracking you), but thats generally different for each site, and they tend to change a lot depending on context. i haven't seen anyone go into detail on how to read these yet, so this is probably a good place to do so
these are called query strings, and they can store literally anything that the website wants (e.g. search parameters, video id, source identifier). each piece of information is separated by an & symbol. for example, here is the url i got when searching "what is a query string":
https://duckduckgo.com/?ia=web&origin=funnel_home_website&t=h_&q=what+is+a+query+string
can be split up into pieces like so:
https://duckduckgo.com/ ? ia=web & origin=funnel_home_website & t=h_ & q=what+is+a+query+string
it's a lot easier to see now, but each piece of information generally has a name, and a value, separated by an equal sign. we can then display our pieces like so:
origin = funnel_home_website
q = what+is+a+query+string
looking at these pieces, you can start to deduce what is needed and what isn't. for starters, "origin" being set to "funnel_home_website" is tracking where i started this search from: the duckduckgo home page. i'm not too sure what "ia" means, but it being set to "web" means it's probably tracking what platform i'm searching from (my browser). i have no idea what "t" being set to "h_" means, but with "q" being set to "what+is+a+query+string", i know for a fact that's my search query. since this is a search website, that's really the only important bit that needs to stay in the url, so i can get rid of all the other pieces, which leaves us with a much shorter (and more readable) url with no trackers in it:
https://duckduckgo.com/?q=what+is+a+query+string
for another example we have this youtube.com link. i've highlighted all the important symbols in the same manner as before:
https://youtube.com/watch?v=dQw4w9WgXcQ&si=Aa1Uc_fRHXC0ay85
looking at it we have two parts labeled "v" and "si", both seemingly set to gibberish. removing both of those gives us a url that does nothing, so if you didn't already know what these parts are, you would have to guess and experiment, since the names and values arent incredibly helpful here. "v" here probably stands for video, and as op already pointed out, "si" here stands for source identifier, so the only important part of this query string is the "v=dQw4w9WgXcQ"
generally speaking, the strategy is to find the piece of information that's important for the site you are currently using (video id/search query in this case), and then remove everything else. websites like youtube try to rely on you not knowing how to read these by making them look like nonsense, so knowing how to weed out all of the unnecessary tracking bullshit is incredibly useful
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