[Idiom] are you up for and are you down

[Sunny1110]

As far as i know the phrase "Are you up for something" means "Are you interested in something", yet I have just watched some video the character said "Are you down for / with ( I forgot) it? " the content was about a guy asking a girl about dating going out or something.

So, Is "Are you down with xxxxxx ? " = " Are you up for xxxxx" = " Are you interested in xxxx?'"

 

[bhaisahab]

As far as i know the phrase "Are you up for something" means "Are you interested in something", yet I have just watched some video the character said "Are you down for / with ( I forgot) it? " the content was about a guy asking a girl about dating going out or something.

So, Is "Are you down with xxxxxx ? " = " Are you up for xxxxx" = " Are you interested in xxxx?'"

I'm afraid your context is far too vague. "Are you down for..." is possible, but it wouldn't have the same meaning as "Are you up for...?"
"Are you down for the trip tomorrow?" = "Have you put your name on the list of people going on the trip?"
"Are you up for the trip tomorrow?" = "Are you interested in going on the trip?"
For example.

 

[susiedq]

"Are you down with . . . " can mean "Are you in agreement with" . . . or "Do you understand . . ."

 

[Raymott]

"Are you down with . . . " can mean "Are you in agreement with" . . . or "Do you understand . . ."

Really? Not in AusE. "To be down with something" means to be ill.

A: "Are you up to a game of tennis tomorrow?"
B: "Sorry, I'm down with the flu."

Note though that "to be down with" and "to be down for" are two entirely different things.