Both words refer to time, but they work differently.

By tells us the deadline.
„Please send me the report by Friday.“
Friday is the latest acceptable day.

Until tells us how long something continues or how much time is available.
„I’ll be working on the report until Friday.“
„You have until Friday to finish it.“

💡 Quick tip:
By = deadline.
Until = continuation or time available.

In business communication, this small distinction can make a real difference.