Every micro radiates and this radiation is of course a function of its design. But this only plays a role, if you let the micro come into direct contact with the environment, means if you have no metal enclosure, no filter at the supply inputs, no filters at the ports, etc. Then, indeed, the internal design (package (!), number of GND pins, turn-off of ALE, PLL-oscillator with spectrum spreading, etc.) plays a role.

But a good design refering to EMC "isolates" the micro from the environment, by shielding, filtering and grounding. And, of course, as Andy mentioned, furtherly makes it immune to EMI by "protecting it against surges, glitches, brown-outs, etc." and also by "software features like watchdogs, debounce, etc.".

By consequently applying the above measures you can decrease the EMI level and the level of susceptibility to EMI to nearly any degree you want.

Kai