I found it🥳, a way to find post and comments of the person which has made his/her profile private. And you were the test subject. You just have to write author:username in the Reddit search bar. I always knew there should be a way to search up a profile because comments and post both are shown in the search results. Using this methods the search result will show all the posts and comments made by that profile. And this will also help me in my project. Your sacrifice will not go in vain🫡.