One of the unique H5P features is the possibility to reuse H5Ps within a site or across sites. This is achieved either through the copy/paste feature or through the file download/upload feature. Both of these features (if enabled), are available by selecting the reuse link displayed below any H5P.
H5P download/upload file
From the first version of H5P, there has always been the possibility to copy an H5P from one site to another. The only requirement is that you need a platform supporting H5P (e.g. Drupal, Wordpress, Moodle, H5P.com and more). The downloaded H5P file contains everything needed to be able to both display and edit the H5P on any H5P enabled site.
Note: The H5P file is not runnable on your local machine (standalone).
The copy paste feature has been available from within the H5P authoring tool since 2018, and in February 2019 it was also made available when viewing an H5P. This feature uses a special per site "H5P clipboard". I.e. you can only use this feature within an H5P site, not between different H5P sites.
The architecture of H5P has made it possible to reuse content types in many different contexts. Multiple Choice, e.g., can be used standalone, but also inside Course Presentations, Interactive Videos, Question Sets and more. Because of this, it is also possible to e.g. copy a Multiple Choice from an Interactive Video, and paste it into an already existing Course Presentation. You can also do the same for a standalone Multiple Choice. The only requirement is that the "receiving H5P" already supports this H5P content type (and version). If not, there will be a message saying "unsupported content type" when you try to paste it.
Note: The "H5P clipboard" is not the ordinary clipboard on your machine. It lies within the web browser, and can only be used in the H5P context.