Difference between domain name registrar and registry explains the fundamental distinction of registrar and registry https://youtu.be/uMSuJ8n2MZ8 .
This video tries to describe the functions of the domain name registrar and registry. Domain name registry is an organization which maintains a record of specific TLD whereas domain name registrar helps the clients to easily register domains and maintains another record. First of all, you need to understand who are they and why are they. If you realize that, then you will say the basic difference.
You cannot skip registrar as they play important role in domain registering. They became authoritative, when you register a domain trough them. They maintain a record in their server and that server is called authoritative server.
Here is a video which explains what happens when you register a domian name https://youtu.be/atlRym3Atms.
Blog post http://websitemaya1.blogspot.com/2017/12/domain-name-registry-vs-domain-name.html