"... [Verb] upon ..."

What difference is there when we add ‘upon’ with many verbs like "… expanded upon…, depends upon… etc when we can use ‘on’ instead ?

Thanks in advance.

It sounds old-fashioned nowadays, except for some semi-fixed phrases (‘depend upon’ is one).

oxfordadvancedlearnersdictio … onary/upon
Although the word upon has the same meaning as on, it is usually used in more formal contexts or in phrases such as once upon a time and row upon row of seats.

macmillandictionary.com/dict … itish/upon
[color=green]FORMAL used after some verbs with the same meaning as ‘on’
A police patrol happened upon a robbery in a bank yesterday.
They draw upon their experience to educate and guide new companies.
My whole future depended upon the decision of this one manager.