Sophia, a classically trained pianist, here at around grade 6 standard, improvises for the first time....
The teacher is playing the second piano part in a way which interacts supportingly with what the pupil is doing and at the same time allowing space for the pupil to do her own thing.
Using the pentatonic scale on black notes is a very easy way to get started if you have never attempted any improvisation before.
Using plenty of pedal helps too.
Explore the many different sonorities you can achieve by making 'gong-like' sounds in the pedal at different dynamic levels - this is something you can do for yourself, and on your own at home.