Apple USB-C to USB-A MJ1M2ZM/A: USB 3.0 or 2.0?

It provides 3.0 service. But the adapter's speed is not your only concern. Generally an adapter like this at best has little to no effect as its a simple passthrough and in this case very short. But the reality is, the more things you introduce between source and destination the greater the potential to degrade the signal.

https://www.macworld.com/article/221042/how-fast-is-usb-3-0-really.html

All of your components matter when it comes to transfer speed and all must be compatible at the same transfer rating to get overall "fast". You will never reach speeds attained in a laboratory environment unless you precisely duplicate the conditions.

Adapters are just that ... adapters. They attempt to match unmatched components. That in itself means it will not help your overall speed as it's a "make do" solution for convenience more than anything.

If you want Max speeds, buy new, latest protocol equipment and forego the use of all adapters

Apple does not follow the "blue insert" convention that many cable and device makers are using.

This is a diagram of a nine-conductor USB-3 port.

A USB-2 port is exactly the same size and shape, but has ONLY pins 1 through 4, and can not transfer data as quickly. The additional pins provide more data pathways AND provide balanced (aka push-pull) drivers that can work over longer distances and higher speeds more reliably.

To support the higher speeds of USB-3, all connectors and cables must have all nine pins present, regardless of the color of the insert.

It's backwards compatible; works with both.