iPhone XS battery maximum capacity

That's normal. Battery capability declines with use.

Make an appointment at the Apple Genius Bar for service. If you need to find an Apple Store - Find a Store - Apple. Also, see How to find your nearest Apple Authorized Service Provider (AASP) or Distributor (AAD).

"Your Apple-branded or Beats-branded hardware product (“Product”) is warranted against defects in materials and workmanship for a period of ONE (1) YEAR from the date of original retail purchase (“Warranty Period”) when used in accordance with Apple`s user manuals (refer to www.apple.com/support/country). Under this warranty, you will be able to direct your claims to Apple even in situations where you purchased the Apple Product from a third party.” [Apple One (1) Year Limited Warranty – Accessory - For Apple and Beats Branded Products Only.]

I may disagree with this. I have an iPhone C which is one year old and current battery health is 94% and my Xs max newly bought has been 100 since October till last month where it went to 99 and in a week 93. Something is wrong...

158/500 = 32%. 32% of 20% (that is, 100%-80%) is 6.3%. 100% - 6.3% = 93.7%. So your phone is tracking almost exactly according to spec.

I just explained that your phone is tracking according to its specifications, so there is no hardware problem with it. How many charge cycles does your friend's phone have?

I can't think of any other way to say it.

YOUR PHONE IS WORKING PERFECTLY. THERE IS NOTHING WRONG WITH IT. WHETHER YOU THINK SO OR NOT.

Sorry typing error, I meant iPhone X