I did know how to create an object's pagelayout and how create a lightning record page respectively, but I have difficulties to add lightning record page into the object's pagelayout.
Secondly, I also confused about assigning the specific pagelayout to a user having the system administrator profile.
It will be greatly appreciated if anyone could help me to answer the question, thanks a lot!
Manoj Nambirajan (Dell Technologies) 
Forum Ambassador
@Yi Liu Pagelayout is the basic foundation of the record that you see. hence for every object as you have pagelayout, we define what all section and fields should be made available.
and then assign that pagelayout to the profiles (like system administrator here).
This is same layout users will see when they create or edit a record.
Lightning record page, acts on top of the pagelayout and you can try few more customizations in the record page. For instance.. dynamic forms, dynamic related lists etc.
Please go through below links to understand more
https://www.salesforceben.com/salesforce-page-layouts-vs-lightning-record-pages/
