What? Herd immunity would involve a vaccine or millions of people dying. The objective, then, is to keep as few of people from dying as possible before we get a treatment or vaccine.
The reason EVERY expert says mass testing is critical is because we need to prevent hospitals from getting overwhelmed, so people can get needed treatment, and slow the infection rate.
The concept of trace/test/isolate is really common in preventing disease spread. This would be just a much bigger version of that:
If you're unwell and get a positive COVID-19 test result, the health department would then theoretically speak to you about where you've been in the last x number of days, who you've been in close contact with, what day you went to the grocery store, etc. They would then reach out to those people/places to inform them and suggest everyone get tested. By doing this you can catch as many people as possible who might have been exposed, encouraging them to get tested. Any with positive results would stay at home, thereby preventing others from getting infected, etc.
The same system has been used for Hepatitis, HIV and TB for many years. If a restaurant has a worker test positive for Hepatitis they generally notify their customers. If you go get an HIV test and it's positive, the health department asks for a list of your sexual contacts to confidentially notify them to get tested before they could infect someone else. Same concept here, just on a bigger scale.
Read this:
The United States has conducted 3.7 million tests for COVID-19.
abcnews.go.com