Default Value of the flex Shorthand Property
The flex shorthand property combines flex-grow, flex-shrink, and flex-basis. When no value is specified, it falls back to its default values.
flex-grow: 0 (the item will not grow by default).
flex-shrink: 1 (the item can shrink if necessary).
flex-basis: auto (the item takes its natural size).
flex: 0 1 auto;
This means by default, a flex item will maintain its initial size (auto), can shrink if the container space is limited, and will not grow to fill extra space.