Power BI Data Flow: Multiple Tables & Relationships Explained

When you have a relationship between two tables, how does the data flow from one table to another in Power BI? In short, how does the data flow across relationships between two tables?
Before explaining multiple tables, I used the same data in a single table and performed the Sum() function on the Sales column. As you know, Power BI generates a summary report; I used Matrix visualization to show the summary result. I explained how this formula works internally in Power BI on a single table. Power BI uses an engine called the x-velocity in-memory VertiPaq engine, which understands DAX formulas.
However, when I divide the same data into two tables, a one-to-many relationship is created between the two tables after importing them into Power BI from an Excel workbook. One Excel workbook can have many sheets. One record from one table can have many corresponding values in another table, which is why it is a one-to-many relationship. This is also called Data Modelling in Power BI.
I have explained how the data gets filtered, how the filtered data flows across the relationship, and how the Sum() function is applied to the shortlisted values in the other table.
After 9 minutes, I also explained the primary key and foreign key concepts of RDBMS because you can also import data into Power BI using any Relational Database Management System, such as SQL Server, Oracle, MySQL, etc. Then, I practically demonstrated step-by-step how to import data into Power BI using an Excel workbook, create a one-to-many relationship, and rename the sheets in the Query Settings in Power Query Editor during transformation.
These relationships are created inside the Model View. Later, I verified the output using the Matrix visualization. Whether I generated the result from one table or two different tables, both gave me the same result.