Can I use my iMac, Mid 2010 running 0S X Yosemite as a display or monitor for my MacBook Air, 2020?

It could work if your Mid 2010 iMac is a 27" model.

For the required adapter and cable, plus the instructions,

see > Use your iMac as a display with target display mode - Apple Support

There are reported cases, where it may or may not work from the MacBook running Catalina.

Then no it will not work, only 27" models supported TDM. We recommend that you buy an external display for your MBA. They are easy to setup and the prices have come way down.

We understand.

Good luck.

Thanks for the follow up RK. I got side tracked running an errand for the wife.

Understand. Hey are you retired now?

I'm of age, but going to hang on for 1 more year to keep the wife Insured.

I'm definitely of age (66) but holding on for another 19 months, my employer will pay 1/2 my medical after 10 years (19 months from now) of service. So I'm sticking it out.