It’s somewhat naive to think in terms of whether you ‘believe mainstream media’. Everyone has their biases & prejudices, including journalists. All big organisations have their blind spots, including media companies. We have to be aware of them and to consume media from a variety of outlets to get a genuinely balanced view.
The big public service broadcasters, eg BBC, Sky, ITV all bend over backwards to be truthful, trustworthy & objective. The problem is that their idea of objectivity may be different to yours or mine.
These organisations are made up of people recruited disproportionately from a very limited range of backgrounds. They are younger, more urban, more liberal, more middle class and much better educated & informed than the general population. Their education is disproportionately in arts & humanities. There are very few journalists / editors/ producers with STEM degrees which is why most media science coverage is dumbed-down, innumerate & trivial. Many BBC people have never worked in the private sector, which is why they don’t understand how businesses work or why profit isn’t a swear word. Most political journalists have never had to make a big decision which affects the lives of anyone outside their own family, which is why they don’t understand the difficult choices & trade-offs politicians face when every major decision they make creates winners & losers. Most society journalists are so terrified of offending certain minority groups that they turn a blind eye to large scale abuses happening in plain sight, eg illegal sweatshops in Leicester, grooming gangs, FGM, forced marriage etc etc.
They are obsessed with ‘diversity’, but only in very narrow terms of ethnicity gender & sexuality. What about diversity of age? Social class? Education? Accent?
The mainstream media presents a worldview which, if you’re a young, liberal, London based graduate makes sense to you. If you’re a retired miner or steelworker or a struggling young mum in a minimum wage job in Mansfield or Middlesbrough it feels like they live on a different planet.