ref – https://www.natterandramble.co.uk/future-perfect-tense-timeline-form-uses/

The Future Perfect tense expresses action in the future before another action in the future.

Future perfect
[will] + [have] + [past participle]

The future perfect shows an action that will be completed in the future by a specified time. Because it depends on another time, the future perfect is often used with words like by, before, at, or when.

By the time you read this, I will have already left.

She will have eaten lunch before her sister even wakes up.

ex:

1) Alice will have returned home.
2) The party is over.

Alice will have returned home when the party is over.

ex:
1) Amelia will not marry Paul
2) Unless he finishes school in the future.

Amelia will not have been married to Paul until he finishes school.

ex:
1) Next July starts
2) We will not have received our degree.
By next July, we will not have received our degree.

ex:
1) Summer starts
2) We will have built the bridge.
By next summer, we will have built the bridge.

ex:
1) Next week starts
2) We will have earned lots of money.
By next week, we’ll have earned lots of money.

ex:
1) Summer starts
2) He will not have received enough money.
He will not have received enough money by the summer.