Expanding/Collapsing columns html code?

I don't know the html code for expanding/collapsing columns (as in eg. there is a spoiler on a site, and they give you an option to see the spoiler by clicking on it where some writing appears on the same page without being redirected [expanding] and then you have the option to close that writing and make it disappear by clicking on it again [collapsing]).

Do you have a site or something that can tell me the html code for this?
Thank you.
 
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