@leocg Yeah, but then why the selection of pre-defined search engines is much bigger on mobile Opera? There I can select Qwant. This makes no sense.

Why it's not possible to add Qwant (or any other search engine) as default?
Seems like their addons have been removed from Opera addons page, and installing it from Chrome store doesn't make Qwant the default - it requiers to use some shortcuts.