English - By x Until

Both indicate “any time before, but not later than”.

Until tell us how long a situation continues. If something happens until a particular time, you stop doing it at that time.

They lived in a small house until the end of December (They stopped living there in December).

I will be out until Friday (I will be back on Saturday).

We also use until in negative sentences.

Details will not be available until January (January is the earliest you can expect to receive the details).

If something happens by a particular time, it happens at or before that time. It is often used to indicate a deadline.

You have to finish by January 20 (is the last day you can finish or you may finish before this date.

We also use by when asking questions.

Will the details be available by January?

Source: Google