How does MySQL specify the starting auto-incrementing primary key ID of the newly inserted data?

In MySQL, when we set the primary key ID to auto-increment mode, for example, the current maximum primary key ID is 8, so how can we specify the primary key ID of the next inserted record as 100, as long as the following SQL statement is used:

Alter table user auto_increment = 100

When we delete some records in the table, we can set a smaller value for this ID. When this value is smaller than the maximum primary key ID of the current record, mySQL automatically uses the largest primary key ID + 1 as the primary key ID of the next record.