I’m afraid I can’t really help you on the QuickBooks database. There is a DB Connector node, but in order to configure this correctly for your database you probabaly need some driver for it.
I googled what and found this: https://quickbooks.intuit.com/learn-support/en-uk/data-and-systems/odbc-driver-faqs/01/239231
It mentions a driver can be downloaded when you have QuickBooks Enterprise. Otherwise you have you buy one from FlexQuarters.
Maybe someone of the KnimeTeam members can help you on this?
For the learning and explaining: over the last 5 years or so I followed many data science courses and did exams.
In april I joined the Spring Summit 2020, which was held online. Learned a lot on how Knime should/can be used for different types of problems.
If you want to get more background on Knime search on youtube for knime (and knimetv). There are many short video’s to explain things. On the Events calender you will see there are regularly webinars held, which you can often join for free. Previous webinars are published as well online. There are courses too, but these will cost you some fee.
So it doesn’t come overnight, but you do the right thing: keep on asking! 