In general, the MED is a "normal" Blacklight (v5) application, with a lot of added stuff.
Like most Blacklight apps, the heart of the configuration is in the Catalog Controller, specifying the search and facet fields. This is repeated for the other controllers, and they're fairly straightforward so long as you're willing to take on faith that Rails will find things at the right time.
The files in models are unusual for a Rails app in that we're not using a backing database, and thus not deriving them from ActiveRecord. The directly is empty of anything interesting.
The actual objects to represent each of the many, many layers of a dictionary entry are actually defined in the middle_english_dictionary repository. The nomenclature can be confusing, since it's derived from the jargon of the field, but none of the objects are particularly complex.
The meat of the interface is actually built into the presenters.
quotes presenter
is indicative of the setup, pulling in lots of XSLT and getting values
out of the document with XSLT transforms or by querying the
Nokogiri::XML document directly.
...is all indexing code, and isn't used in the rest of the application. See indexing.md for a brief overview.
...has all the solr configuration in it, as well as the Dockerfile and the container initialization code. See solr.md for more.
...is basically just normal views and utility objects (most importantly the services.rb file).