Difference between wifi card and on board wifi chip

What is the difference between a wifi card and on boaed wifi chip ? I have a motherboard with built in wifi chip (intel ac 3168 dual band) and find something quite interesting. After reducing it transmit power and radio frequency to 2.4ghz,
games, programs, bios and srivers download speed reduce but seem to have better lossless quality.
In simple words, it's like a more faithful copy of the source. I guess it's because a built in wifi chip does not
perform better than a dedicated wifi card. What do you think of it ?

There is no difference other than the on board MOBO WiFi adapter cannot be upgraded, however you can always get a PCI-E Wireless Card and use that instead and possibly upgrade later.

Both have external aerials, they have to as they are in a metal box.

That choice is yours.

There is no basic difference between 2.4 GHz and 5 GHz other than allowable ERPs, lower losses for 2.4 GHz, more channels used in the 5 GHz band.

WiFi has a lot more transmission errors than an Ethernet cable, so data has to be re-sent.

I know these are simplifications.