How children's of flexbox got the full height without having height property

in this challenge, I have noticed something strange. let me explain in details
there is a container having the display flex property and it has a height of 600px, and the container has 2 children and they took the full height of their parent how that happened and there is no height property on them

challenge link

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flex has a default of


Which is saying whatever in the flexbox stretch it to the full height. modify these properties and you can see the difference