In MySQL 8.0.18 there is a new feature called how it works under the hood. The high-level basics are the following: if there is a join, it will create an in-memory hash table based on one of the tables and will read the other table row by row, calculate a hash, and do a lookup [&]

https://www.percona.com/blog/2019/10/30/understanding-hash-joins-in-mysql-8/