You install some VST plugins, you do stuff, re-install your DAW, maybe change DAW, and one day you realize that you haven’t got a a clue where your precious plugins are installed.
It took me a while to 1) understand that there are two types of VST plugins (VST2 and VST3), and 2) where I should look for them. Or maybe 3) if you count that plugins can be 32 or 64 bits.
The VST2 standard is old (the latest 2.4 spec is from 2006 and Steinberg discontinued maintenance in 2013) but there are still many plugins around. You recognise the VST2 plugin by the file extension .dll.
The VST2 standard does not have a dedicated installation folder. VST2 plugin installers usually let the user select an installation path. But VST2 plugins can also have a fixed installation path predefined by the manufacturer. In this case, only the documentation can tell you where to find the plugin.
Here are some common VST2 plugin paths:
C:\Program Files\Common Files\VST2
C:\Program Files\Common Files\Steinberg\VST2
C:\Program Files (x86)\Steinberg\VstPlugins (32-bit plug-ins on 64-bit Windows)
The VST3 standard was introduced in 2008, but since VST2 worked fine, it has taken time for manufacturers to switch to VST3 (if they have at all). Since 2013, Steinberg have stopped maintaining VST2.4, but this is not a big problem, your DAW will support it for many years still.
The VST3 plugin has the extension .vst3. The VST3 format has a dedicated installation path all VST3 plugins must comply with, so all your VST3 plugins should be installed here:
C:\Program Files\Common Files\VST3
C:\Program Files (x86)\Common Files\VST3 (32-bit plug-ins on 64-bit Windows)
VST plug-in locations on Windows https://helpcenter.steinberg.de/hc/en-us/articles/115000177084-VST-plug-in-locations-on-Windows
VST 3 Locations / Format https://steinbergmedia.github.io/vst3_doc/vstinterfaces/vst3loc.html
VST 2.4 vs. VST 3.0 – Who Cares? You Should. (Craig Anderton) https://www.sweetwater.com/insync/vst-2-4-vs-vst-3-0-who-cares-you-do/