This is correct - Cantabile is a program that can host VST2 dll plugins.
Grace is a specialized VST instrument: it is a sample player. Remember the horribly expensive hardware samplers that dominated the sound of the 90s (Akai S3000 or the EMU IV come to mind)? These were hardware boxes costing north of $5000 that allowed you to sample natural sounds, map them to keys and play them back. Grace is essentially the same (just without the ability to actually sample, since converting audio to WAV files is easy nowadays) as a software program - and instead of costing $5000, it comes for free!
BTW: we don’t usually talk about VST2 "dll"s but simply about VST plugins and VST instruments (shortened to VSTi by some). So Grace is a free sampler VSTi, available as a 32bit and as a 64bit VST2 plugin - you need to download the correct version for your needs. Then you use the Grace manual to understand in detail how to load WAV files in Grace and map them to keys.
Almost correct, but not 100%: you need Windows 10 64 bit version to run 64 bit programs like Cantabile 64 bit (and of course the 64 bit plugins as well). There is also a 32 bit version of Windows 10 - that will only be able to run Cantabile 32 bit (and 32 bit plugins). BUT: Windows 10 64 bit can ALSO run 32 bit programs - so if for some reason you only want to run 32 bit plugins, you can run Cantabile 32 (with 32 bit plugins) in Windows 10 64 bit! Windows 64 bit has some technology built-in that allows you to do that…
But the rest of your statement is correct: when running Cantabile 64, you need to run 64 bit plugins or use JBridge so Cantabile 64 can “understand” 32 bit plugins.
Most commercial plugins today are available as 32 bit AND 64 bit plugins, but a lot of the free ones (especially the ones built with some older plugin construction kits) are only available as 32 bit versions, so they won’t run in 64 bit hosts, unless you bridge them.
I’d recommend leaving that for later and just start with a basic set of VST instruments and other plugins in one technology only, either 32 bit or 64 bit and get going with the corresponding version of Cantabile. Once you’ve mastered the basics, you can move on to the question of bridging or other more sophisticated wizardry.