What do the various USB 'current' labels under macOS System Report mean?
The USB section of the System Report actually provides data for each of the USB devices connected to your Mac.

The information you're asking about relates to each of those devices. More specifically:


Current Available is the standard amount of power (in milliamps) provided by the specific USB port
Current Required is the amount of power (in milliamps) needed by whatever you have plugged into that USB port - if nothing, it should display 0 (i.e. zero)
Extra Operating Current refers to a special feature that Apple Macs support in relation to other Apple devices whereby under the right circumstances they can provide additional power (i.e. above the standard amount) to those connected devices. The right circumstances are as follows:


The Mac must be running and booted into macOS (i.e. not booted into another operating system)
The Mac must be awake (i.e. not in sleep mode)
The connected device must be manufactured by Apple (I'm not aware of an exhaustive list anywhere, but typically the device is an iPod, iPhone or iPad)
The connected device actually requires the extra power for some reason (i.e. the Mac does not provide the extra power unless the device actually requires it)



As far as the maximum power output available, this depends. By default the USB 3 ports of your iMac (the ones you're referring to as USB-A) offer up to 900mA. However, for this to occur you must be using a USB 3 cable. If a USB 2 cable is being used it will limit the available power current to 500mA. And, as mentioned above, the amount of power offered in the case of some connected Apple devices can exceed these figures under the right circumstances.

As for the Thunderbolt 3 (USB-C) ports, these can supply 15W of power per port to external devices. They can even be used for charging MacBooks (although they're not intended for that purpose and it's not something I'd recommend, plus the rate of charge will be quite slow).

[UPDATE]

Below is some further reading from Apple's website that you may find relevant:


If a Mac accessory needs more power or is using too much power
Using USB devices with your Mac
About the external features and ports on iMac computers introduced in 2017 or later