How to charge AirPods to keep battery healthy? Once on the official Apple website it was written to keep iPhone's charge between 20% and 80% for better battery life.

I've not found exact battery specs for the AirPods (I am about 2nd gen), probably they have the same battery type as latest iPhones (Li-ion).

So, do I have to keep AirPods with at least 20%+ charge while using? I can't keep them 20%-80% because of case will charge them even they are at 99% battery.
... do I have to keep AirPods with at least 20%+ charge while using?

No.  Use the AirPods for what you purchased them for - wireless headphone with Siri control.
Lithium ion batteries don't need to be micro-managed like previous generation batteries needed to. They don't develop a memory and will function (generally) for their usable life.
The key to this is to understand that age and cycle count is, by far, the largest factor in determining battery health.  You can mitigate factors that will diminish battery life like avoiding extreme temperatures and physical trauma but there's no procedure or practice you can do that will extend the life of the battery.

I can't keep them 20%-80% because of case will charge them even they are at 99% battery.

This is also not an issue and a misunderstanding of how charging works.   Batteries and chargers are &quot;smart&quot; in that they will automatically reduce/stop charging when it reaches a full charge.  Batteries naturally discharge without use, and the charger will kick back in recharging the battery.
The good news is Apple has official battery service options now and some third party companies are also competing in the battery repair service space.

https://9to5mac.com/2021/02/09/what-to-do-with-old-dying-airpods/