по большому счету принципиальной разницы между использованием on и upon нету. они равнозначны. что же касается "up on" - оно может использоваться только для обозначения местоположения. ну и есть ряд усиоявгихся выражений, где используется только один вариант, например "once upon a time", "to be on schedule" и пр.

Upon us used in fixed expressions and optionally instead of ON in certain expressions (as Igor pointed out). It’s considered somewhat literary and is rarely heard in colloquial speech. UP ON is used when something is in a high place on top of something: “He put the cake up on the refrigerator so the children wouldn’t get to it.” “The dictionary is up on the shelf.“

There’s also the expression “to be up on something,“ which means to be familiar with something, or up-to-date on something like fashion, music, etc. “My brother is such a techie – he’s up on all the latest gadgets.“ “Read this report so you’ll be up on the case before the trail begins.“

Agreeing with the comments above.

"The boy is up on the elephant" = "The boy is up and the boy is on the elephant" - The boy is on something that is above the speaker or above the boy's normal position.

"Big fleas have little fleas upon their backs". The little fleas are on top of the big fleas but the big fleas may be anywhere.

"to be up on something" as Uly mentioned is about up-to-date knowledge.

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Всем спасибо 👍🏻