When you use USB-C/Lightning wired earphone,

The DAC (digital-to-analog converter) & amplifier are built inside the headphone cable/earpiece housing itself that converts digital → analog

Those digital circuits can generate local electromagnetic noise near your ear.

When using an adapter, the DAC sits entirely inside the dongle, away from your head.

The output is analog only, flowing through copper wire to your earphones.

That's why using 3.5mm adapter > using earphones with built-in DAC