You can show trend lines in a visualization to highlight trends in your data.
To add trend lines to a view, both axes must contain a field that can be interpreted as a number. For example, you cannot add a trend line to a view that has the Product Category dimension, which contains strings, on the Columns shelf and the Profit measure on the Rows shelf.
However, you can add a trend line to a view of sales over time because both sales and time can be interpreted as numeric values.

When using a Blend, the primary data source appears with a BLUE tick-mark and the secondary data source appears with a ORANGE tick-mark.
See below:

A URL action is a hyperlink that points to a web page, file, or other web-based resource outside of Tableau. You can use URL actions to create an email or link to additional information about your data. To customize links based on your data, you can automatically enter field values as parameters in URLs.
Read more in depth at : https://help.tableau.com/current/pro/desktop/en-us/actions_url.htm

According to the official Tableau documentation:
An example of an area chart is shown below:

According to the official Tableau documentation: