How to or where to begin even designing a scrolling-animation website mockup like...

You could design such things in Figma, but you can't build a fully functional website. I have heard good things about Framer.com - a good nocode/low code builder that focuses on good scrolling effects and animation. Its been a while since I last used it so I don't know if many things have changed. There is also webflow.com which is similar.

On Figma, you can do a prototype with some animation - it's pretty decent animation. Good to use when you want to design and share. But that won't be the functioning end product.