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An interesting article about iPhone Antenna design for 5G:

https://www.macrumors.com/2020/02/14/apple-5g-iphone-antenna-module-custom/

Some notable quotes:

As Fast Company points out, Apple has run into problems with antennas designed internally before. The iPhone 4, for example, had an antenna design that resulted in dropped calls and other problems when the iPhone was held in a way that covered the antennas. Fast Company's source says that another recent Apple antenna design "required twice as much power as comparable antennas to produce the same amount of radio signal."

Basically the last sentence means that the antenna design in question would have 3 dB lower antenna efficiency than typical antenna designs. I would guess this would be the wifi antenna, as cellular antenna efficiency is more tightly specified by the cellular carriers.

Another quote:

Creating 5G antennas for mmWave networks is harder than creating other kinds of antennas because they send and receive higher frequency signals, leaving less room for error. 5G performance is also reliant on the antenna design.

To be fair, all cellular performance (2G, 3G, ...) are also reliant on the antenna design - how else do you think these things are wireless? 5G antennas are much higher frequency (28-39Ghz in this case), so are more susceptible to minor changes that can influence performance. However, one advantage of these antennas (known as mm wave antennas) is they are more localized within the device, you don't have to worry about distinct parts of the device causes efficiency losses to your antenna (as is the case at say 800MHz).

Basically, because 28-39GHz antennas are small, they can be integrated directly on top of the Chipset (IC). This can be good for implementation, but the chipset vendor would then require a certain amount of metallic keepout around and above the antennas. Apple wouldn't want anyone telling them where to put metal in their product, so them designing their own antennas would make sense.

Apple has historically been willing to give up a few dB of antenna efficiency for a nicer looking product.