reference pages for the META SEO Inspector Google Chrome extension.
about viewport meta tag
Tells the browser that this webpage is optimized for mobile, changes the logical window size used when displaying a page on mobile browsers.
width=device-width to match the screen's width in device-independent pixels, and please ensure your page is accessible by not disabling user scaling.
The presence of this tag indicates to Google that the page is mobile-friendly, this is very important as Google gives prioriy to this kind of pages in its SERPSs. Don't forget to build the page using responsive design to make it really mobile-friendly.
Without the vieport meta tag, the First Input Delay (FID) could get lower scores.
The advised parameters for a mobile-friendly page is the following:
<meta name="viewport" content="width=device-width,initial-scale=1">
The exception for not allowing page scaling is when a page is a PWA (Progressive Web App) as they mimic native apps behaviour.
<meta name="viewport" content="width=device-width, user-scalable=no" />