Thank you to Gandalf_The_Grey for posting the information about user agent switcher (I had known previously about the nano extensions situation).
I had user agent switcher in a browser, and have now uninstalled it. Thank also for posting the links about infected extensions.
Because of this, I have now decided to add all my extensions in chrome browsers via load unpacked. To do this I will download the crx and put the file in a zip archive - unzip the extension into a folder and on the extensions tab turn on developer mode - and then add the unzipped files through the load unpacked button. If I want to update, I will download and inspect the latest crx, and load it as unpacked.
I have decided to do this after reading the link you posted "Help for Users Impacted by Infected Extensions" - where the writer explained that fraudulent malware operators have been posing as anonymous new developers asking to purchase the rights to the extensions. Then they update it and put in malicious payload codes, and add the update to the chrome web store - and then chrome automatically updates the extension with the malware in it - without any user knowledge or input.
So, as stated, from now on I will only add extensions through the load unpacked button - and manually install new updates after inspecting the updated crx file added to the chrome web store - although this process will take longer.
A side note - some chrome browsers do not auto update extensions - iridium is one of them.
Thanks again.