WordPress Connecting to a Database

I’m new at WordPress so please bare with me.

My software company is in the financial industry and we have customers that then have their own customers.

I have been tasked to setup a website using WordPress so that our customers customers can log into their account to view their information, and maybe even make changes.

My question is, can WordPress deliver this type of product? If so, how or where in WordPress do I connect my site to our database and extract data from it to display on my WordPress site?

Is it as simple as creating the WordPress site and using Javascript?

Can someone recommend a book or website with this level of detail? This is way beyond writing a Blog and I’m afraid WordPress may not be capable of doing what I’m looking for.

Thanks everyone.