What Is Their Main Difference?
The main difference between 'by' and 'until' is that 'by' talks about deadline while 'until' refers to all the time between the action's starting point to the end of the action.
Talking about Deadline
We use 'by' to talk only about the deadline. When we use 'by' we are emphasizing the time that an action or event came to an end. Have a look:
Your assignments must be on my desk
I need to be home
Talking about a Period of Time
We use 'until' to talk about the time between a starting point and the end of the time period. For example:
I have to work
The Zoo is only open
With Other Prepositions
'Until' can be used with other prepositions to give more detail about the time period. Have a look:
Our work days are from 9 a.m.
The ice festival is open from December
Are They Interchangeable?
'By' and 'until' are not interchangeable. Replacing them with each other will completely change the meaning of the sentence.