Does Pages support LaTeX extensions? Is there a location where I can install LaTeX plugins so that I can use the commands in the plugins which are not native to LaTeX in the equation editor in Pages? Or does Pages not support that?

Specifically, I have the mhchem package which I'd like to install to write out chemical formulae – a feature not supported natively and is added by the mhchem package. 

(Yes, I'm aware that I can just write the documents in LaTeX directly, but I'd prefer writing it in Pages if at all possible.)
Pages does not support loading LaTeX packages (they're not called plugins, but packages in LaTeX terminology). So you cannot just load up for example chemfig from CTAN that you mention.

Apple provides the following page describing the commands that are supported:

https://support.apple.com/en-us/HT202501

Pages does not include a full LaTeX environment, and is only able to render a subset of LaTeX commands.

For my own purposes I have used ordinary LaTeX rendered to PDF that I can drag-n-drop to Pages. As it is PDF it can be resized inside Pages without loosing quality.

In order to do that you need LaTeX installed. An easy way to get it installed is to install MacTex, which includes LaTeX.

You can download MacTeX from here:

http://tug.org/mactex/mactex-download.html

Or if you have HomeBrew, you can use that to first install HomeBrew Cask and then install MacTeX using the following commands:

brew tap caskroom/cask

brew cask install mactex


For rendering to PDF you can use the pdflatex command or a GUI tool such as texpad.