Best charging practices for MacBook Air M4 battery health

Should I start charging from a certain percentage to maintain battery health, and is it okay to unplug it when it is between 80- 100%?

Plug it in when it's convenient to do so.

Also, is it harmful to my device or battery if I use it while it’s charging?

No.

Use it the same way as your phone, your electric toothbrush... etc. Apple actually thinks about these things, so you don't have to.

Batteries - Maximizing Performance - Apple

About battery health management in Mac notebooks - Apple Support

The best practice is to plug into power whenever it’s convenient and run on battery only when you cannot connect to power. Your Mac uses Optimized Battery Charging, which is enabled by default. That is designed to charge the battery in a way that balances increased battery lifespan with your personal usage patterns. 

About battery health management in Mac notebooks - Apple Support

To confirm that Optimized Battery Charging is enabled, go to System Settings > Battery then click the (i) to the right of Battery Health.

If your Mac is usually plugged in, macOS will pause charging at 80% full since that’s better for the battery long term. If you frequently run on battery, macOS will charge the battery to 100%. That behavior will change depending on your usage. For example, I use my work MacBook Pro connected to a dock for most of the week, and charging pauses at 80%. If I run on battery a few hours for a couple of days, it will start charging to 100%. If I then keep the Mac plugged in for a couple of days, it will again pause at 80%. When your Mac decides to hold the battery at 80% charge, it will bring it to that level even if the battery is fully charged and the Mac is connected to power, which is what one of my Macs did here:

If charging is paused, you can override that by clicking on the Battery icon in the Menu Bar or Control Center (you may need to enable that in System Settings > Menu Bar) and select Charge to Full Now.

Once ‘trained’ the system is pretty responsive. For example, when my Mac was holding at 80%, a couple of short (30-45 minute) sessions running on battery one morning was sufficient to have macOS decide it was time to bring the battery to a full charge.

The bottom line is that your Mac will manage its battery charging to preserve the health of the battery while supporting your usage patterns. There's no need to micromanage it, let macOS handle it.

Open Settings->Battery. Under Battery Health, click the little circle with an "i' in it. If not already turned on, enable Optimized Battery Charging.

Then, let the MacBook Air learn your habits of usage and it will manage the best battery charging scheme for use.

Then just use your MacBook Air normally and just don't bother fussing over micromanaging your battery charging and simply enjoy using your Mac.