This is a question of labeling to meet users' expectations. Will they be disappointed if they tap on "Help Center" and there is no toll-free number to call, or chat agent available? Will they understand immediately that "Knowledge Base" is entirely self-service?
Help Center sounds active (because "Help" is both a verb and a noun). The label sets the expectation that, while there may be reams of documentation available, there is also someone standing by on headphones, waiting to actively help me with my task. I might expect to see a link to initiate a chat, a phone number in giant display type, a search bar to do a little self-service help, and a list of top questions people ask (FAQs). With a Help Center, I may be able to find a quick answer to my question, or I may need to chat with an agent to resolve the problem. Either way, I am likely to succeed.
Knowledge Base, on the other hand, sounds passive because there is no verb in the phrase. It implies that this is more than a collection of links; that powering it all is a [knowledge management system created according to best practices], and there may be no human assistance available.
The melding of the two ("Help & Knowledge Base") implies that there may be a knowledge management system available for self-service, and full-service help options as well, via chat or phone call.
For your purposes, you might be best served with a "How To Use This App" or "Get Started with App" link and save the Help and Knowledge Base for when your needs have grown past the basic Quick Start modules.
: Aiken, Best Practices for the Implementation of a Knowledge Management System in Small and Medium Enterprises (https://scholarsbank.uoregon.edu/xmlui/bitstream/handle/1794/21960/Aiken2016.pdf)