On chrome on desktop, it's easy to view a website as it appears when viewed on chrome on many other devices. That is, simply cmd + option + j to open dev tools, then clicking on the 'Toggle Device Toolbar' lets you select which device you want to view the site as. For example, this is stackoverflow.com viewed from chrome on desktop (it can be made to show what would be seen by a user accessing the site from chrome on an iPhone 8):
How do I view a website as seen from other devices safari running on desktop?
I've tried changing the user agent, which simply tells the website to serve what it normally would to a mobile device, however, this doesn't affect the fact that it's still being viewed on a desktop browser, which is very different to a mobile one.