There are four main ways to add blocks from within the Gutenberg editor.
By hitting enter, you automatically create a new paragraph block (which can be changed)
Hovering your mouse at the middle-top of any block brings up an “insert block” button.
Clicking the upper-left “plus” button adds a block below your currently selected block.
After you’ve hit enter to add a new block–you can then select a few commonly-used block options (located on the lower-right of the block)