To add a new Line
To adjust number of decimals during labeling
In the Label Class Expression window display the field that would be used to label.
More on the math functions in Arcade here – Math functions | ArcGIS Arcade | ArcGIS Developers
Add thousand separator
Similar situation here as in the example above. But this time, on top of formatting number of decimals we want to add thousand separator to long numbers. Instead of rounding the number, we would convert into a text and apply formatting:
Additional reference: How To: Add Thousands Separators to Labels in ArcGIS Pro (esri.com)