There are 41 "Core" modules in Backdrop CMS 1 "Core".  They are located at:

BACKDROP_ROOT/core/modules

The modules are listed here, in alphabetical order.  

Administration toolbar module (admin_bar)

Block module (block)

Book module (book)

CKEditor module (ckeditor)

CKEditor 5 module (ckeditor5)

Color module (color)

Configuration module (config)

Contextual links module (contextual)

Comment module (comment)

Contact module (contact)

Contextual links module (contextual) Drupal 8 Contextual links Documentation

Date Module (date)

Content Translation module (translation)

Dashboard module (dashboard) *

Dblog module (dblog)

Email module (email)

Entity module (entity)

Entity reference module (entityreference)

Field module (field)

Field SQL Storage module (field_sql_storage) **

Field UI module (field_ui)

File module (file)

Filter module (filter)

Image module (image)

Installer module (installer)

Language module (language)

Layout module (layout)

Link module (link)

List module (list)*

Locale module (locale)

Menu module  (menu)

Node module (node)

Number module (number) **

Overlay module (overlay)

Path module (path)

Redirect module (redirect)

Search module (search)

Syslog module (syslog)

System module (system)

Taxonomy module (taxonomy)

Telemetry module (telemetry)

Testing module (simpletest)

Update module (update)

User module (user)

Views module (views)

Views user interface module (views_ui)

* This module was initially removed and then completely re-implemented using a different code base from the Total control module (total_control)

** Sub-module of Field module