I started the modding journey a couple of days ago myself, this is what I can share so far.

https://github.com/hovgaardgames/startupcompany/wiki
This place is a good start that I completely missed out on until now, most of my time was spend in the development console inspecting elements and doing searches in my IDE where I had opened C:\Program Files (x86)\Steam\steamapps\common\Startup Company\electron\resources\app for any hits on the things I was looking for. By the looks of it I wasted quite a bit of time since there are quite a few helper functions listed on the wiki.

GetRootScope() lists a lot of useful stuff. There's also Game.Lifecycle that has a couple of useful things that have to do with employees.

Since it's all Javascript and html the modding isn't all that difficult, grab a mod or 2 from the workshop that don't seem too complex and look through the start.js file and the html views (if they have them) to get an idea how it's build up.

What kind of mod are you wanting to make?