I used dribble.com and Pinterest.
I searched for one page Website UIs and turned them into HTML and CSS templates.
Here's how it works
After finding the UI i already have few things such as, Color combinations, and overall Layouts
After that i find a similar font from Google fonts to match the one same in UI design
For images i use Unsplash, pixels is a good Alternative
After gathering everything into 1 folder i try to code the ui as same as possible
Every ui on internet can be remade or is made using flexbox or grid. For starters you can learn flexbox and then go to grid as that makes things easier
To learn anything you'll need to practice it more and more. I've used flexbox enough that i have confidence using it to make UIs and you can do it too