This question is not about “how to install”, is about:
“how to acces?”, “how to trigg?”, “where de menu?”, “whre the button?”
any answer that shows how to use after install.
PS: at Wiki, nothing about, https://wiki.documentfoundation.org/Extensions#How_to_use_Extensions
It depends on the extension; language tools like dictionaries integrate themselves into existing tools; many extensions add a menu-entry in one of the existing main-menus; and some extensions add a new entry to the main menu.
An example could be the (included) MediaWiki-extension; in Writer, to activate it, you choose “File” → “Send” → “To MediaWiki”.
I have the Anaphraseus-extension installed for translation work: it has its own main-menu entry and a small toolbar, wich I place where it pleases me.
Best is to read documentation/manual for each extension.