This might be pedantic, but iirc email clients don‘t execute any javascript due to security concerns (at least that was the case when I worked in email marketing 7 or so years ago).

The javascript is basically only used for templating, defining reusable components and rendering the static HTML together with inline CSS which does make a lot of sense because writing „bulletproof“ email components that work on every client is a huge amount of work and you would generally like to keep the amount doing that to a minimum.

In my experience every company uses some type of email templating system, some off the shelf, some built in-house, I actually got my start in (actual) programming by building one for my team.