Why is my MacBook Air charging till 100%. I want to he stops at 80%.

Rule No. 1 regarding battery care: don't obsess and try to micromanage things. Let the OS and hardware do their job.

The best advice for getting the most from your battery:

• Plug in when using the computer near available power.

• Use on battery when ac is unavailable.

• Don’t subject the computer and battery to extreme temperatures.

• Let the OS and charging circuits manage the charging.

• Try not to allow the battery to discharge deeply regularly before plugging in. Yes, it will happen from time to time, just don't make it a habit.

• Don't obsess over battery life and/or health.  

Here is Apple's guidance on the subject.

Batteries - Maximizing Performance - Apple Support

About battery health management in Mac laptops - Apple Support 

Alexmath998 wrote:

So it's better that I just let it charge and let it do its thing? Because most of the time it sits on my desk and it's barely flat.

Yes. Leaving it plugged in all the time will not harm your computer. Just let it do its thing.

Yes. Let the OS and the charging circuits do the job they have been designed to do.

Don't worry about something that isn't a problem... until it's a problem.

Enjoy your new(?) computer and use it in good health, my friend. 🙂

❝Worry is the misuse of imagination.❞ – Zadra

D.I. Johnson wrote:

❝Worry is the misuse of imagination.❞ – Zadra

I love that!

I live it. 🙂