Where can I find the vst plugin I’ve created?
dll was the extension of the old VST2 plugins, which you are not allowed to create anymore except you signed a contract with Steinberg some years ago that allows you so. In that case you should also have a copy of the Steinbergs VST2 SDK which you will need to build them.
If you are new to the plugin game you have no other choice but building Plugins against the new VST3 standard, which is supported out of the box by JUCE. The resulting bundles have the extension .vst3 – maybe that’s what you are looking for?