1. In general the plugins downloaded will be with the extension opar and when you have done the 188.8.131.52 OEM install, it deploys the default plugins which gives the compatibility to monitor its Weblogic, DB, Host targets.
2. All the plugins deployed will be residing in the Plugins directory, these were the plugin binaries. If you have a Disaster Recovery, we have a different procedure to sync the plugins. In case of additional OMS, we can directly deploy them from console.
3. The agent software downloaded will be in the software library.
In 12C you can get the software using Self Update console (Setup tab > Extensibility > Self Update).