Will a company I am applying to contact my previous employer without my knkwledge even if I select don't contact previous employer?

If you are in the states, all they can tell them is "yes she/he was employed with us". Not your personal info. Not your pay info. Not even that you were/were not a good worker. If they follow the laws. If not and it costs you employment, sue!

If this is a corp, most don't even contact your previous employer other than confirm employment. Not even sure why some still have that question. Obviously, you're looking for a new job for a reason. In addition, we were told under no circumstances to say anything about a current or former employee because of possible liabilities. So I could put down, sure you can call my former manager, she is unlikely to say anything and defer to HR who won't say anything other than you worked there.

Some don;t even ask for references anymore. After all, you're not going to list a person who is going to say bad things. What's the point.

So, how do we find out things? Well, if there is someone else working at the place you're applying from your current or previous employer and they know that, they will ask.

Do not contact my previous employer is red flag. If I see it on a CV, it’s getting tossed.

They could - although it would be horribly unprofessional

Most likely.