This is just an adapter, not a DAC.

Realtek ALC 5686 is the DAC chipset. It is a mistranslation on the MFG specifications picture as 'usb codec'.

Not actually a mistranslation! The Realtek ALC5686 is a stereo codec, which is a combination of DAC(s) and ADC(s) in one chip package for audio out and mic/audio in.

Some codecs include a headphone amplifier as well, such as the Wolfson WM8731.

This thing has a Wolfson WM8731? Wow that’s impressive.

Needs more description then.

No the Wolfson was another example of a stereo codec, the codec used in this is an ALC5686

But it really improves the sound right? If so, I plan to use it with my iPhone

But can’t seem to find an American retailer.

Yes it likely sounds better than whatever came with the phone.
I have the Hidisz USB-C (Sonata HD I think?), Which sounds better than the included adapter, but not as good as my dragonfly, as an example. There's a bit more to it than just the DAC though, the rest of the circuitry around it defines how it sounds, a 'good' DAC is as transparent as possible.

Yes, should be better.

After this review I've noticed that it is easy to play louder than I should be doing. This is probably because there is less distortion as volume rises.

Never liked the Dragonfly much, so I’ll try this. Help me find an American retailer

The issue with audio is we all share different perceptions. One might say the Dragonfly makes the iPhone sound so much butter, but I say it barely made any difference (tried both Red & Black). Now a real breakthrough improvement in audio came when I tried the Chord Mojo. That was a game changer, where I noticed a significant improvement over the iPhone’s built-in DAC.

No idea how the ddHiFi TC35 compares with the Mojo.

I've never heard the Mojo so I couldn't say. Given the form factor, I doubt this approaches what the Mojo can do.

Yes, it's better than most phones 3.5mm output. But no it's not going to be better than much more serious self powered DAC/AMP units.