It doesn't happen. It cannot. Journalism always contains some bias. I have written for a living since 1996 (books, journal columns, features, and more recently, training courses and presentations) and I can confidently say that, from my experience, journalism is never unbiased.
We know that the national newspapers have their own overt biases. When we pick up a newspaper we know their political leanings (they all tell us who to vote for at the General Election, so of course they all have a political bias). We also know, in general, how a newspaper will react to a news story and we make allowances when we read the article and subconsciously strip out the opinion and bias to extract the news. (However, to be frank, the Daily Express support for Junior Doctors surprised me).
Journalism always has a bias. Journalists on the Nationals usually excuse themselves by giving a quote from someone who is known to have an opinion contrary to the main opinion they are reporting. I find that lazy and inadequate. A formulaic quote from a Press Officer at a government department, or a public body, can never be regarded as a balanced response. For a start, a Press Officer is, by definition, detached from the issue, they are not responsible, nor are they necessarily knowledgeable of the subject. A Press Officer is simply another journalist, poacher-turned-gamekeeper: a journalist giving a quote to another journalist. Worse, a Press Officer is merely parroting the statements from the person who the article's journalist should have interviewed, so why haven't they?
Some journalists have suggested that any subconscious bias in their copy is removed by editing. I find this hard to believe, particularly if an article is written by a "specialist journalist". As their title suggests, specialist journalists are unique at the publication. Other journalists editing the piece will not have adequate knowledge of the subject to be able to determine if there is "bias" in the article. At most publications, the editing that occurs is more about the publication's style guide, grammar, spelling and legal liabilities, than about the subject of the article.
Most of my career has been writing for technical journals. I have never used the term "reporter" to describe what I do, and I rarely use "journalist". The reason is because I wrote columns and features and refused to report on the carefully created press releases, or press conferences, from the vendors of the products I wrote about. More recently, I have worked for those vendors and have written such press releases (or "white papers" as the vendor insists on calling them) and feel justified that I ignored them when I wrote for the journals. I always called myself a "writer" because I knew that everything I wrote was my - well researched - opinions, and I was employed because the journals' editors trusted my opinions.
These journals always had a rather unhealthy relationship with the dominant vendor because although they were subscription publications, and had some income from their readers, a significant proportion of their advertising came from the dominant vendor. Thus, the articles were about the products of the provider of the journal's main income. That is a difficult situation to manage and can lead to a form of hagiography, or worse, simple churnalism where press releases are regurgitated. This is not how I worked.
I always made sure that whenever I wrote an article I always went back to first principles: try the product myself and write about what I experienced, and not what the vendor said I should experience. Further, if I was critical - which I usually was - I would say why there was a problem with the product, but more importantly, how the reader could work around the issue. The vendor was generally happy because I was helping them to improve their product and provide ways to keep their customers using their product. I could not do this by merely reporting what the vendor said because the vendor rarely wanted to publicise flaws in their product.
Sometimes (but not often) I would be contacted by people who worked for the vendor and they would indicate issues they were concerned about. Whistleblowing is an overused phrase, but I guess it's possible to say that such people were "whistleblowing" on the product. A journalist on a national newspaper would have reported such issues as "concerns from an unnamed source". I never did this, because I could see that there was nothing to be gained by reporting second hand concerns. If an issue was reported to me, I would replicate the issue, sometimes spending a large amount of time writing code to illustrate the issue and its cause. I would then publish my findings and how to replicate the issue. I would never jeopodise a source by quoting them as "an unnamed source" and I would never write an article based solely on the opinion of someone unwilling to be named: I would always do the research myself first,
My opinion is that all written words are biased to some extent, and that it is impossible to remove all bias. Worse, any attempt to "remove" bias by so-called balancing opinions with bland statements from Press Officers is amateur and lazy. It is far more important that a journalist clearly states their biases so the reader can take them into account when reading the article, than to attempt to remove (in their opinion) any traces of bias from the report.
You may have detected the use of the past tense when I talk about writing for journals. The reason is that a few years back I wrote a critical piece about the most significant product of the dominant vendor. As always, I provided the results of my research and the tools for readers to replicate my findings. After the article was published, I was contacted by a senior manager at the company who asked me to retract my findings, and gave me an incentive, he offered contract work to help them improve the product. It turned out that the manager was not authorised to make the offer of work (and so it was a hollow promise), and anyway, I later learned that a manager several levels up in the company had declared that I was not welcome at any of the company's sites and my name was removed from the list of writers would got preview copies of their products. In effect, my career writing for journals was shut down.
Ironically, after that I worked for a publishing company where I wrote conference presentations and "white papers", for the vendor who had "banished" me. My name never appeared on these items, so the manager who had declared me persona non grata was none the wiser, and was likely to have given one of my presentations.