Synonyms and also formal/ informal. You have to distinguish in some cases like if you are in danger, you would call for help not assistance. If you are at the airport with elderly and need a wheelchair, you would need someone to assist you…to need assistance

Depending on the context they can mean exactly the same thing. Although assistance is a noun and help is a verb as well as a noun. Assist being the verb form of assistance. I would say assistance and assist and more formal than help.