i want to buy iPhone XS Max how to do it?

Oops, I missed the "XS". For the iPhone 8, 8 Plus, Xr, or 11 series:

1. Go into any Apple store.
2. Go to https://Apple.com
3. Go into any Apple Authorized reseller.
4. Go to the website of any Apple Authorized reseller.

Not that only 1. and 2. will sell you a carrier-unlocked phone. All other sources will sell phone linked to a specific carrier, or, in some cases, to any carrier in a specific region, but the phone will lock to the first carrier it is used with.

Apple no longer markets the iPhone XS. You may be able to obtain one from an Apple store, or other authorized dealer.

You’re welcome.