Microphones are high impedance
Dynamic are 200 to 600 ohms or so, electret mics are typically a few k due to the FET buffer load.
I wouldn't call that high impedance - in audio 47k is the standard load for line signals for instance.
Most line-outs are ~100 ohms or so due to the resistor on their outputs which serves to prevent
oscillation driving capacitive loads. However most opamps driving line outs aren't able to handle
heavier than 1k loads without overload distortion appearing at higher signal levels, so its wise
not to load a line-level signal heavier than about 2k if you care about SQ.
Driving a headphone from lineout will create quite significant distortion at higher volume levels as
the opamp's output current limiter operates, unless its a high-current opamp designed for the purpose,
or a very high impedance set of 'phones.