what connector is best for Mac 2021to use USB plugs

I had not heard any stories of adapters causing issues such as you describe. You could check at a retailer such as Amazon or any big box department store. You could also check out the Apple adapter at:

USB-C to USB Adapter - Apple

You will need a USB-C cable or USB-C to USB-A adapter to connect existing devices.

For the mouse, you will need to enable the “Secondary Click” option within system preferences.

You are quite welcome.

"How do you right click on the mouse to get copy, paste etc. This new mouse doesn't do that."

System Preferences -> Mouse -> Point & Click -> tick Secondary Click