How database buffer pool work?

For example we have 2 scenarios:

Scenario 1:-

We have very small buffer pool size of 100MB. My query data is 500MB. Does the system keep 500MB query data in RAM such that a new query will read from RAM?

Scenario 2:-

Buffer pool size is 1GB. In this situation will the system always keep 1GB free (in RAM) for MySQL query processing. Can the 1GB be used by other processes?