Can you please explain to me how a play or movie could be full of allusions to someone like Shakespeare?
Do they give credit to these famous people or just make fun of them?
The film is full of allusions to Hitchcock.
A play full of allusions to Shakespeare.
As StoneyB comments, the name of a famous artist (playwright, novelist, movie director, etc.) is often used as a synecdoche (a part representing the whole, or the whole representing a part) for the [collective] works of that artist.
So if someone says "I studied Shakespeare at school" it's quite possible they were never taught anything at all about the man himself, and his personal life (of which relatively little is known anyway). It's almost inconceivable they studied all of Shakespeare's works either - it's quite possible in such contexts to use the artist's name when in strictly literal terms you actually just mean one thing produced by that artist (which itself "stands in" for his total output, as a "typical" example).
Taken to extremes, thousands of people claim to have "studied Homer" - but so far as I'm aware we don't even know enough about Homer to say whether one could meaningfully identify him as a single person.
And as oerkelens comments, "allusions are basically just references". They might be positive, negative, neutrally informative, artistically creative/evocative, etc., depending on context.
