@leocg I believe that's why, a pity that there is no way to change the UA, I really like Opera mobile, but with a small problem, it's bad to use.

@leocg Yes, it is set to mobile
My user agent in Brave and other chromium based browsers:
Mozilla/5.0 (Linux; Android 12) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.5060.134 Mobile Safari/537.36

My user agent in Opera:
Mozilla/5.0 (Linux; Android 12; 2109119DG) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/102.0.5005.134 Safari/537.36 OPR/70.3.3653.66287

As we see above, in the UA of other browsers it appears "Mobile Safari", in the UA of Opera only "Safari", I believe that this causes the sites to be displayed in Tablet or PC version.
I have two devices, Poco F3 and Xiaomi 11 lite 5G NE, this problem happens on both, whether Opera common, Opera GX or Opera Touch, the only exception is Opera Mini.

@stockchannel The truth is that apparently almost no filler app works on Android, not even Google's

I used Opera for years, but over time and with the growth of the smartphone screen, I have been facing this problem since 2019 in Mi MIX 3 5G, when I open the browser it understands that I am on a tablet, the interface changes for tablet (so far ok, I can change it in the settings), the problem is websites that it displays on desktop instead of mobile pages, when checking the UserAgent on website, I saw that it really understands that the device is a tablet, contrary to what happens in other chromium based browsers.
Is there any solution for this? I thought about changing the UA, but Opera doesn't allow this in the settings.