Primary Elections

I read an article in The New York Times (https://www.nytimes.com/2021/07/21/opinion/house-elections-constitution.html?searchResultPosition=3 ), which stated:

"But since the advent of the primary system in the early 20th century, members of Congress often have to face two elections every two years."

Prior to the early 20th century, how were party nominees chosen? What caused the change? Could we go back to the prior system?

I asked able2know about this previously, but this article suggests that the answers I received were wrong.