TYPO3 v8 has reached its end-of-life March 31st, 2020 and is not maintained by the community anymore.
You can order Extended Long Term Support (ELTS) here: TYPO3 ELTS.
The Extension Manager (EM)¶
Extensions are managed from the Extension Manager inside TYPO3 by "admin" users. The module is located at "Admin tools > Ext Manager" and offers a menu with options to see loaded extensions (those that are installed or activated), available extensions on the server and the possibility to import extensions from online resources, typically the TER (TYPO3 Extension Repository) located at typo3.org.
The interface is really easy to use. You just click the +/- icon to the left of an extension in order to install it and follow the instructions.