The same way a tailor understands stitches even if a sewing machine creates most of the stitches a tailor will ever stitch. Because understanding code is tangential to writing it. Which isn’t to say that writing it doesn’t help solidify knowledge and certainly plenty of people learn by doing and there may even be skills that atrophy as a result of not writing code in the same way that skills for writing assembly have atrophied with the use of higher level languages. But ultimately it is possible to understand some code without having written most of it by hand.