what are <nav>, header,<article >,<footer> used for

Answer 1

nav is used for a navigation bar

header is used to place a header

article is used to place unrelated or slightly different content (from main)

footer is used to place a footer

Answer 2

Header is a tag used to place a header

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https://www.sololearn.com/learn/HTML/2199/
https://www.sololearn.com/learn/HTML/2195/