Plugin versus Module
Yes, you're right: Modules are still modules, the equivalent of what WordPress calls plugins. Each module can implement one or more plugins.

For example, the Voting API module has a plugin for each of the methods to show the result: Average, Sum and Count.

Plugins also needs a plugin manager, which is the responsible for actually calling the plugins.

You can see this on:


https://github.com/pedrorocha-net/votingapi/blob/8.x-1.x/src/Plugin/VoteResultFunction/Average.php
https://github.com/pedrorocha-net/votingapi/blob/8.x-1.x/src/VoteResultFunctionManager.php