Let's imagine that you are working as a web designer for a start$-$up that needs its website to be visually appealing and responsive to different screen sizes and devices. Your go$-$to tool in this case could be the Flexbox model that would enable you to design a flexible layout fitting various screen sizes without needing to use float or positioning properties.

Explain how you would utilize the flexible box layout model to design a responsive webpage for the start$-$up$,$ considering factors such as alignment, direction, order, and flex$-$direction properties.

One of the main characteristics of Flexbox is its ability to alter its items' width$/$height to best fill the available space. How can this characteristic of Flexbox influence the overall aesthetic and functionality of the web design especially concerning user interactivity and experience?