Followed a Pinterest link here, seems interesting so I joined. Might seem like a dumb question, but: Why would I buy a DAP when I have my music on my IPhone?
There is absolutely no NEED to buy a DAP. If you buy your music on iTunes then it can be a pain to even move that music to a DAP. An iPone and airpods pro deliver a very convenient listening experience.
But what if you want to step up?
Bluetooth is bandwidth limited, the Apple lightning to 3.5mm adapter also limited although to a slightly lesser extent and not all high-res services and formats are easily accessible on iPhone.
The question then is what direction do you want to go?
If keeping the convenience of the iPhone is a requirement then look at a headphone DAC, a fiio BTR5 (or any of its competitors) might be a good start. Now you can get the music out of the phone at the quality of the source. You get the convenience of bluetooth for the situations you need it and connected via a lightning to USB-C cable it will drive most IEMS and the easier to drive headphones.
Keeping with the iPhone but with harder to drive headphones then there are larger portable DAC/Amps's like the ifi hip-dac, fiio Q5, xduoo xd-05 (my choice). DACs at this level are useful beyond the phone and are stepping into the desktop/hi-fi headphone amp area while still being portable/luggable.
If you don't need to keep your music on the phone then DAP's become relevant. Even entry level DAP's will maintain the source file resolution, many offer balanced output, are capable of driving all but the most demanding headphones.
DAP's tend to offer more storage space out of the box than you can spare on your iPhone with many DAP's accepting micro-SD cards.
More and more DAP's are adding wireless features letting you access streaming services when you have wi-fi, and can go on to offer more advanced music management features to the point of becoming a digital music hub for your home.
So like so much in life "you pays your money and makes your choice". You will keep paying more and more money though. Everytime you think you have the perfect setup you will realise that you are not one step closer to the answer, just one step further from the question.