With expressions, you can perform operations on strings, dates, and numbers. You can convert data from one type to another, perform mathematical operations, parse data, and manipulate data to obtain what you need.
There is a second tab next to the Dynamic content tool, called Expression. The following screenshot shows the Expression tab:
The following screenshot shows an expression that concatenates the first name and last name:

You build a flow by adding steps, taking the output from one step and using it as the input for another step:
Before looking at using Power Automate with Common Data Service, we need to learn about the different ways to initiate flows using the different types of triggers.