изменить display: inline-block;
на block
для .pp
класс
.pp {
height: 400px;
width: 100%;
display: block;
background-color: red;
}
.p {
height: 200px;
width: 100%;
display: inline-block;
background-color: aquamarine;
}
.flex {
margin-top: 100px;
height: 50px;
width: 100%;
display: flex;
align-items: center;
justify-content: flex-start;
background-color: blue;
}
<div class="pp">
<div class="p"></div>
<div class="flex"></div>
</div>
crowded