AdSense account approval process

Some context: I am tired of the corporate game. So, I, along with a team of 4-5 people, are building a SaaS website related to files and its operations for eg. converting media, pdf etc. I want to earm from ads basically to start with.

I want to understand from someone who has got its AdSense account approved for a website. Like what all things i should keep in mind before applying? What is the TAT? What are the rejection reasons?

Any type of help is going to be super helpful to me. In return i can address your technical queries, if you have any.