In Site Builder, there are two different ways to create line spacing when using the text editor. They are known as soft and hard returns.
1.) Soft Return - A soft return occurs when you reach the end of a line of text and the cursor "wraps" to the next line without creating any extra space between said lines of text. To do this manually, you press both the shift + return/enter keys on your keyboard. This will place the next line of text immediately under the existing line of copy without additional vertical spacing.
Soft Returns used in Site Builder
2.) Hard Return - A hard return is used to create a new paragraph in a block of copy. The resulting new paragraph will contain extra spacing after there first line or block of copy. This is accomplished by simply using the return/enter key on your keyboard. The next line of copy will contain additional vertical spacing, noticeably separating it from the previous line of copy.
Hard Returns used in Site Builder
Both techniques can be used to create consistent line spacing when editing text in Site Builder.
For more information on hard and soft returns click the link below: