checkupdate.mainsourceofupgrade.best yeah right, that sounds legit.
This works I think because we've all been well trained to keep Flash up to date.
The most obvious symptom of the infections I've seen is that your default search engine is changed from Google to Bing. I've no idea why that might be done but perhaps the browser extensions it installs is able to intercept your searches to Bing to better target ads at you.
To check for, or get rid of, this unwanted software, I recommend Malwarebytes which will let you do a scan for free and only asks for money if you want it to remain and keep scanning. This seems very fair to me.
I ran it on my machine and am clean, my neighbour had 17 bad things.
If you choose not to purchase then you can remove it via the Uninstall item in the Help menu.
The malware I've personally seen is "Adware" which seems to install browser plugins to show you more or different ads. It's annoying but the terrifying thing is that once on your computer who knows what it might get a look at.
Apple's macOS is getting more secure with more limits on what applications, including browsers, can access and I support this, despite occasional pain with apps that have to ask for lots of permissions.
Be very skeptical of popups on web sites, or even emails, that ask you to authenticate to install anything.