It’s how you’re born!!! My cousin was born left handed back when it was thought that using the left hand predominantly meant you were possessed by the devil. My aunt was determined to make him right handed. She made him take things in his right hand & whenever she saw him using his left hand, she’d force him to switch it to the right hand. He’s now 60 years old; & he is left handed as even with ALL her efforts, she couldn’t change how he was born.
I was born right handed. When I had my stroke, I lost the use of the left side of my body. The doctors said the stroke had been on the right side of my brain & that the right side of the brain controls the left side of the body & the left side of the brain controls the right side. So, technically speaking, your brain controls which side is your dominant side.
While in high school, I had an injury to my right hand where I couldn’t hold a pencil or pen so I couldn’t write. It took a while, but I did train my left hand how to write when needed. Still, once my right hand healed, my dominant side took back over & I no longer fell back on the left hand for writing!!!
My Grandmother was ambidextrous & she had NO dominant side. She did things equally well with her right & left hand. Now that you’ve asked the Q, I’m wondering how her brain determined what she was getting ready to do.