How can i improve the sound of Iphone 11?

I know you have an Apple but what I did for my Android was install UAPP app. There is also an app within that app for a "Bit Perfect" app. Then you use an OTG USB cable. 

The UAPP makes the Android's DAC better. Perhaps there is a similar app for an Apple phone? 

I have a Helm Audio Bolt DAC that I have been pretty happy with. It's a USB C dongle DAC that I use with a Samsung S21. With an android phone I'm using USB Audio Player Pro to stream Tidal and Qobuz and I have no complaints. 

I intend to go this route for my iPhone 11 PM. iPhone--lightning to USB C--THX AAA Onyx. The Onyx decodes MQA.

I'll be using this https://www.amazon.com/gp/product/B09CFZZ8TJ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1 low cost, Lightning to female USB C adapter. 

My only worry is the known issue where the iPhone's Lightning port supplies little to no power to attached devices. 

Portable DAC is big improvement. Personality I use AQ Dragonfly Cobalt and very happy with it. Also if you using Apple Music hi res service you can improve SQ with the phone Settings. Go to Phone Settings then to Music where in Audio section you could play with Audio Quality, EQ, Dolby Atmos etc.

Happy listening.

I struggled with this question for a long time. Traveling with amps and a DACs. It was heavy and bulky or a mass of interconnections.

My solution was to switch to highly efficient headphones designed to be driven by low power. I bought a pair of Ultrasonde Edition 8 that sound great driven directly from the iPhone.

The best solution depend on the circumstances you use the system. In particular, the ambient noise you will be experiencing. I enjoyed this solution for many years. But I realized I was fine leaving “high end” sound at home and traveling in comfort (+200,000 miles a year) was more important and switched to Bose QuietComfort. Not highly accurate but really enjoyable (like your favorite inexpensive wine as opposed to a $250 bottle of Bordeaux) to listen to and incredibly comfortable and could be worn with noice cancelation only in airports and airplanes. 

So, depends on your usage.