In formal written English there are very few uses of the ampersand "&." Typically you should always use "and." But there are some instances where the ampersand is acceptable. For example in abbreviations for example AT&T. Another usage is when referring to Mr. & Mrs. You may also use the ampersand when referring to a business, for example, a law firm as in Baker, Thompson & Myers. Other than these cases you should use "and" in proper written English.
Of course the ampersand is commonly used in informal English for example, texts, informal emails, notes etc.
Hope this helps.