It is probably best to think of the phrasal verb "hit up" as a completely new verb. It is different from the single words that are in it: "hit" and "up". "Hit him up" is an idiom that means to call someone or to ask someone for something, probably money. It no longer has any connection to the concept of hitting. But it will lose that meaning if the "up" is no longer part of the verb. Here is an example: "I hit him up into the air." This sentence shows that "up" is being used directionally as an adverb, telling where I "hit" him, because the preposition, "into the air," describes the word "up." The verbal phrase "hit up" no longer works here because the thought, "I asked him for money into the air," makes no sense. Think of them as completely different verbs. The situation is the same with "clean" and "clean up", even though they have the same meaning. I can think of one difference: "clean up" sounds better in certain sentences. A mother who wants her son to stop cursing might say "clean up your language", but she would not say "clean your language." Both sentences mean the same thing, but one sounds better. I agree with Joseph. Phrasal verbs need to be learned individually. They are vocabulary words to memorize. If I were learning them I would add another flash card for "hit up" as if it were a completely different word from "hit". English has a lot of verbal phrases. I am sorry about that. I hope I answered your question well. I did my best to explain it the way I would want someone to explain it to me. Blessings.

Phrasal verbs can be very confusing and need to be learnt individually. It is not possible to make assumptions about when they can be used.

Look up "phrasal verbs" on the Internet. You will need to do much research and practice to learn these.