Also, I trust more about a professional film critic [...] rather than a simple film enthusiast who wrote his opinion from a keyboard and not from a magazine or/and in an interview.
"Professional" in the context of criticism is a meaningless adjective. In recent decades it's held even less weight, distilled down to just "someone who gets paid for it", full stop. It's not analogous to, say, a high school football player versus a professional NFL player — where "professional" actually implies a separation of skill level in the position. Nor does "professional critic" imply a type of artisanal approach to the craft.
The vast majority of professional film critics featured in popular circulations and the corporate blogosphere have no background in either film study or criticism — — neither of those are a requirement for the job. They largely come from journalism and digital communications programs, sometimes even dualing in business.
Professional film critic jobs are acquired and dictated by the person's ability to meet constant suffocating deadlines, cover the most recent productions/trending topics (to better generate revenue via ad-space and/or web traffic), and maintaining their masthead's rapport with industry reps through junkets and promo features. The content and critical merits average professional pop-journalist reviewer are, more often than not, afterthoughts (if they're seriously considered at all).
The professional critics who are able to successfully market themselves through a variety of publicity efforts (independent of their editors or otherwise) have a better chance to secure exclusive staff or column positions for popular outlets — even then, success generally depends on the person's PR savvy.
But increasingly since the turn of the century — even moreso after the Web 2.0 environment was normalized in particular — the idea/goal of professional movie critics (i.e., pop-journalists) achieving household-name status, or even being the dictating voice of their publication, has become quite rare.
However, there are
many excellent film critics who self-publish while also contributing to commercial media outlets. There are also
many excellent critics who largely just self-publish. Then there's the ocean of valuable criticism in scholarly/longform fields, in which a lot of great self-published critics (including some renowned, ex-"professional" commercial people) spend some of their time.
A good chunk of the greatest and most significant film critics of the 20th century started out as "simple film enthusiasts" writing their opinions in notebooks and publishing them with a small group of like-minded individuals.