How do I get rid of some kind of hijacking device?

MalwareBytes got rid of some stuff, but the way SIP protects Safari, it can't remove some items even if it knows they're there.

Open Safari's preferences and click on the Extensions tab. Disable any you don't recognize. If the problem is gone, click the button to remove the extension(s) responsible.

In the System Preferences, check to see if there's an icon for Profiles.

Well, if MalwareBytes didn’t find it, you can always keep looking with something else. I wrote a little diagnostic program to help show what adware is installed. Download EtreCheckPro from https://www.etrecheck.com and run it. Create a new reply and use the "Notes" tool below to add your EtreCheck report.

If adware is installed, EtreCheckPro will help you remove it, although you may have to supply a password. If you aren’t comfortable with that, just post the EtreCheck report here and other helpers can tell you exactly what files need to be deleted and how to do so.

Disclaimer: EtreCheckPro is my own app. EtreCheckPro is free to use but has in-app purchases available. Downloading EtreCheckPro or using it could give me some form of compensation, financial or otherwise.

It sounds like you have installed adware on your Mac.

Try downloading from the link below and then run it. It is a safe method to remove such adware.

https://www.malwarebytes.com/mwb-download/

Go into System Preferences and see if you have any Profiles installed. Sometimes adware installs profiles to override system settings. Just delete the offending profile if you have one.

There's no reference to it under the Extensions tab? Hmm, etresoft may have a better idea where to look.