If it has no tests, then I would slowly try to build tests to document the functionality that I need. In your case being Angular that might be having simple html pages with the smallest module that you need.
How to find things? If you're on Windows try AstroGrep http://astrogrep.sourceforge.net/ to quickly search and jump around in the code or in any system I use VS Code for a similar functionality. Also learn to use command line find/grep.
The book "Working Effectively with Legacy Code" also helped me be more comfortable navigating and changing large code bases, in a long term view I recommend this book to every developer https://www.amazon.co.uk/Working-Effectively-Legacy-Michael-...
Lastly, I would raise this because the company might not be aware they are buying a low quality framework that maybe ticks all the boxes in the contract but is in effect impossible to use by their current developers (you), it might be there's other people with more experience in said niche that might be able to help. In the private community maybe some people would be able to accept a short contract to help train you.