To be able to accomplish that without using script, all the items (h2,p,p,ul) needs to see each other, which technically mean they need to be siblings.
And then, for wider screens where they sit side-by-side, they need to be reordered, where the h2 comes first (order: 0) and so on.
I used one media query, but you might want to add more, to match more of Boostrap's break points.
To add i.e. an outer border, you need to use a combination of border-top/border-left/border-right/border-bottom on the elements, which also need to be altered with the query's, to cover both horizontally and vertically stacked items.
Updated codepen
@media (min-width: 768px) {
.content {
display: flex;
flex-wrap: wrap;
justify-content: space-around;
}
.content > * {
flex-basis: calc(50% - 30px);
}
.content h2 { /* 1st row */
order: 0;
}
.content p { /* 2nd row */
order: 1;
}
.content p + p { /* 3rd row */
order: 2;
flex-basis: calc(100% - 30px); /* as only 1 `p` in markup, it need 100% width,
or add an extra empty in the markup */
}
.content ul { /* 4th row */
order: 3;
}
}
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Updated, based on a comment at a duplicate question
If one have many items, one can group them, i.e. 4 per group, to make the CSS much smaller.