IO-related wait types in SQL Server: Io_completion and pageiolatch_*

Source: IO-related wait types in SQL Server: Io_completion and pageiolatch_*

A large SQL statement operation, the execution plan contains a merge join operation, observed that SQL long time in the io_completion wait state, if it is to read the related table data, the server should do its best to service, but the server's physical IO is far from reaching the bottleneck.
What the hell is this io_completion doing? Is the table's data page IO request still in other operations? If so, what's the difference with pageiolatch_*? If not, what type of operation is it?

Io_completion This wait type represents a variety of synchronous read and write operations in the data file that are not related to the table and are read from the transaction log.
Occurs while waiting for the I/O operation to complete. This wait type typically represents non-data page I/O. Data page I/O completion waits as pageiolatch_* waits to appear.
The biggest difference between io_completion and pageiolatch_* is that io_completion is a non-data page wait, and pageiolatch_* is the IO waiting for the data page.
Io_completion This wait type represents a variety of synchronous read and write operations in the data file that are not related to the table and are read from the transaction log.

The following scenarios are included:
1. Read log blocks from the transaction log (during any operation that causes the log to be read from disk-for example, recovery)
2. In many operations, such as recovery, DB startup, read from disk allocation bitmap (e.g. GAM, SGAM, PFS page) during recovery
3. Write intermediate sort buffers to disk (called "Bobs")
4. During the merge join, the result of writing to disk or reading the merge join on disk
5. Read-write eager spools (a behavior of data caching to disk) to disk
6. Read the VLF header information from the transaction log.

Reference: https://www.sqlskills.com/help/waits/io_completion/

IO-related wait types in SQL Server: Io_completion and pageiolatch_*