InDesign pixel dimensions do not relate to website sizes

Strange, I havent had this problem. I use Indesign al lot for creating website designs and social media banners.

If you set everything as web-based and use pixels all InDesign items should match your screen.

I know Figma or XD have big advantages over InDesign, but when you are used to a certain ‘Chocolate-application’ it is hard to switch. InDesign has a lot of functionality which is in fact perfect for designing for web. 

Okay I've made a 'web' doc in Illustrator – same scaling issues. But the same size doc 1920px in Photoshop looks like a 1920px website width.

Why is InDesign and Illustrator scaling web documents, so they look much bigger than they actually are?

As someone who has used InDesign for well over 20 years and designed websites for almost as long, if you want to give a mock up to a web designer, draw it on the back of napkin and be done with it. There is no such thing as a pixel perfect website and you'll never get the feel of a responsive site using a page layout application.

I can tell you from experience this will not end well if the client is very picky and they'll be complaining left and right about the fact that the site doesn't look the same on the web as your InDesign mock ups.

If you wish to ignore this advice, go right ahead. I'm not going to even bother trying to explain the fake pixel counts in InDesign.