How to make connections

Can someone tell me how to make connections between different doctypes.
For example: I can see in the employee checkin connections that it is connected with shift type.

How can I make connections like these for custom doctypes?

What you’re referring to is called a Link Field. Check out the following documentation and see if it helps:

How can I give Doctype access and User permissions?

Please check the documentation and video resources once:

Also I just wanted to know that how I can process salary with 2 basic salaries as salary component.
For example:

We have 2 basics salary for 1 employee, 1st is the ACTUAL SALARY which we are creating and the 2nd is the Contractual Amount which we are transferring through Bank accounts and rest of the amount from the Actual Salary is provided as Cash in Hand.

I need your expertise in this to put it in ERP system. So that we can process both at the same time.

Found any solution for this?