This is an oligopoly. Because companies are arbitrary constructs, it's a defacto monopoly by multiple sellers. The terminology is useful, but not in the sense that it is then end-all of qualification.
Yes, oligopoly is a fine word to use, but you can't just wave your hands around to transmute that into monopoly.
Collusion often results in this, which is not a dejour monopoly, but a defacto monopoly. Collusion isn't even strictly necessary. Endstage capitalism naturally selects the winners who stay on top. It's a simple concept taught in beginning economics.
"big companies" == oligopoly == monopoly is exactly what people mean when talking about worrisome "big companies", without the commensurate analogies.
A monopoly is about perspective which is about scale not some subjective definition (like sanderjd endlessly trolls about). Legal systems all over the world combat real/threatened monopolies (eg the CCP recently acted unilaterally) in various forms. Whether you think it's a useful reasoning to group companies and use a specific term is irrelevant. I sincerely hope you can understand that.
Good luck with whatever.
When modelling monopolies economists posit one seller that can set the price because it's simple. Similarly, perfectly competitive markets have an infinite number of competeing firms.
In the real world, monopoly has never been as black and white as that e.g. if someone was granted a monopoly on selling salt, that didn't immediately strike anyone selling salt with a lightning bolt, it just meant you couldn't build an actual business on it since you could hit legal issues.
Similarly, people accused of being monopolies argue that the "market" is bigger (do planes and trains compete for long distance travel?) in a way that is fuzzy and human.
But the bigger issue is how do you tell the difference between someone who doesn't understand something, and someone who refuses to understand something? People pretended that climate change wasn't happening for 3 decades and counting to protect big corporations, so really there's no limit to what their influence can buy.