Your mobile phone itself holds an identification number that is duly passed on to any application or website that requests it.

If you are talking about IDFA (Apple) or Advertising ID (Android), you can disable/block that as a user and then you will be free from all tracking. Any other unique identifiers require additional permissions on both platforms and are not easily obtained.

You can’t find that at all on the web, and I don’t know where on earth you got that from. In this regard, your browser (yes, Chrome too) is more secure than the native apps on both iOS and Android. There’s no way you can find your MAC address, IMSI/IMEI, serial number, or any other nonsense on the web. The only way to do that on the web is through fingerprinting. The data you can use for fingerprinting is also more limited than in native apps.

You get such a warning when sites do something that the law doesn’t actually want.

If only that. However, you are obligated to display such a sterile logo as soon as you use cookies at all. It doesn’t matter if they are only functional, as soon as the cookies are there, the logo is there too, because you have to inform users about your cookies.

Log in to a website? Cookie required. Remember your shopping cart? Cookie required. Remember if someone wanted light or dark mode (regardless of their OS setting)? Cookie required.