Beside means literally “next to”. “Besides” is generally in american usage, if I’m not mistaken, and just means “anyway” or “in addition” or sometimes a type of “it’s not really of importance” The keys are beside the phone. He can do a lot more besides. Besides, he didn’t like those jeans anyway

Beside is for location. My computer is beside the coffee table. The kitchen is beside her bedroom. Besides means “in addition to” usually some information previously given. (I like hunting...it’s relaxing...) Besides, no one’s getting hurt. I could get a piece of paper that means I studied formally. Besides that, I don’t have a reason to go to university.

Ha it depends on how you are using it! But yea if you are using it to mean like “in contrast to some other option” then not much of a difference between beside and besides.

“Beside” is more of a location. Example: “I am standing beside a tree” We also have a figure of speech, “He was beside himself” or “I am beside myself” that means you are feeling a very extreme emotion. The saying seems to have come from the idea of having a feeling so strong that the spirt or soul comes out and is beside you (https://learningenglish.voanews.com/a/how-can-you-be-beside-yourself-/4977962.html) “Besides” is used to mean something like “in addition to..” or “other than this..”. Example: “What do you do besides study English?”