How to make curved lines on the front end? [on hold]

I've been browsing through a lot of UI designs on Dribbble to get some inspiration for my front end development skills.

Most of the designs are absolutely beautiful, but I'm not sure how to implement those designs into a real webpage, because it's much easier to design something when you don't actually have to code it.

Take these design for example:

1

2

How are the curves made on the background?