How about late 2009 iMac model USB ports on back side>

Here are the specifications for both the 21.5-inch and 27-inch Late 2009 iMac's. Each model has four USB 2.0 ports which cannot be upgraded to perform at USB 3.0 speeds. I am not aware of any adapters that would convert Firewire 800 to USB 3, but maybe other posters have suggestions that would work.

https://support.apple.com/kb/sp576

https://support.apple.com/kb/sp696

What is it that you are trying to accomplish? USB 2.0 and USB 3.0 are interoperable, so are you simply trying to take advantage of the faster throughput available on USB 3.0? What type of device are you trying to connect?

You best option may be to look for an external hard drive that supports FireWire 800 which you have on your iMac. In order to make it more future proof consider one that supports multiple interfaces. Here's one example:

https://www.amazon.com/LaCie-Rugged-Triple-External-Portable/dp/B016A4YA3M/ref=sr_1_3?qid=1570739859&refinements=p_n_feature_three_browse-bin%3A668384011&s=pc&sr=1-3

Huh?

Your question doesn't make sense.

Writing an effective Apple Support Communities question

If you are asking about what USB speed, the USB ports on a Late 2009 iMac are USB2 (up to 480Mbps). The first iMacs with USB 3 ports (up to 5GBps) were the Late 2012 models.