Hi @Ilia and welcome!
You can have a look at previous comments in this forum: Where to start learning the Senaite code?
Senaite is developed on Plone, so you could start reading about Plone and Zope (there are some tutorials on the Internet).
Also, Senaite uses a modular approach, so functionalities are segmented into different (Plone) addons. Try understanding an addon and learn its structure: https://github.com/senaite/senaite.databox
The customizations you want should be done inside an addon for your case, from there you can modify Content Types and functionalities from Senaite core and Health.