what is the difference in write under [head] or [body ] in html

Answer 1

the head section contains things like links to stylesheets and such. the body tag has the purpose like the name sais, to be the body of an html document. inside of the body tag are all visible things you see on a website.
while you could also change body and head and it wouldnt make any difference, there still is a purpose for this structure: googles algorithm looks at specific tags and elements to detect the content on a website.

Answer 2

what you write in the head doesnt show up in the browser unlike codes you write in the body section. so basically the head section is there for the browser to read only reason it contains things like links to CSS and Javascript, the meta tags etc.