Very simple beginner SEO question

I am just starting in the SEO department and i read all i can about it, but could not find the right answer for this.
The site i am building and planing to rank, i am doing by importing dummy data on existing wordpress template and changing the content and design as i go. Still the website half made as it is, has a lot of content from the dummy data that is not changed.
Now my question is when planing to send the side for indexing how to prevent google robots from indexing the dummy stuff? I saw that on some pages when you see their blabla.com/robots.txt they stop the google bot with a command like "prevent robots.txt /wp-admin", probably even if you delete all the dummy data front end, there is still some data left on the backend
Is that the corrent way to do it or is their another catch?
10x for whoever takes the time to answer.

If you edit your robots.txt that should prevent the pages with dummy data to not get indexed. I dont undrstand what you mean by some data on the back end. But editing the robots.txt should do it.

yes just go into robots.txt and do what you said except add the directory of the post or page

prevent /name-of-page

10x for the answers. For the dummy data on the backend i thought of some pages or posts created with dummy data content, who you can no longer see on the front end, but are still present in the backend

I'm sure there must be other ways, but no one really indulges in that because the robots.txt method has been tried and tested, and is really simple to implement. You should be good to go with some simple edits.

you can make the posts/pages private