text-size-adjust vs viewport meta tag

So, I'm designing a mobile-first website with the chrome mobile simulator. I saw that when I put some text in it, for example a <p>, it will display very very small, and will not take into consideration the font size that I gave it, unless the <p> contains several lines of text. I did some research and I figured that this behavior is due to the text inflation algorithm used by mobile browsers and it can be stopped by using text-size-adjust:none. Is that correct? Then, the viewport meta tag. I figured that the viewport meta tag is used when a website it is not designed mobile first, and will help display the content properly based on the smaller width of a mobile devide. Is that correct? In conclusion, text-size-adjust with the value of none is used when building a mobile-first website in order to stop that inflation algorithm and help display the text properly, and the viewport meta tag is used when bulding a NON mobile-first website. Is that correct? Thank you