You can also insert a block using what's called the 『slash command'. When your cursor is sitting in an empty Paragraph Block, you can convert that block to another type of block by typing a forward slash (/) and then the name of the block. As you type more characters, the list of blocks being displayed will filter down based on what you've typed. For example, typing 『/im' will show you a list of blocks that either start with, or are related to images, such as the Image Block, Gallery Block, Media & Text Block etc… If there's more than one block showing, you can either type more characters to filter the list further until there's only one option, or your can use your cursor keys to select the one you want and hit Return/Enter to select it, or simply use your mouse to select the block from the list.
You can also insert a new block using the Add block icon below your content. If the last block in your content isn't a Paragraph Block, then you will see an empty Paragraph Block at the very bottom of your content.
It's important to note that if the last block in your content is a Paragraph Block, then this empty Paragraph Block at the bottom of your content, wont appear by default and you'll need to either hit your Return/Enter key in your last Block for the empty Paragraph Block to appear, or use one of the other methods mentioned here to insert a new block.
If you see an empty Paragraph Block at the very bottom of your content and you simply want to add a new paragraph of text, then all you need to do is put your cursor in the empty block and start typing. If you want any other type of block though, you need to click the Add block icon () on the right-side of the empty Paragraph Block, or use the 『slash command』. Clicking the Add Block icon will show the Block Inserter popup, which is a small panel that gives you access to different types of blocks that you can insert. By default, it only shows six recent blocks. To view all the available blocks, click the Browse all button in the popup to show the Block Inserter panel. Alternatively, you can start typing its name in the search field to filter the list of blocks shown. Once you click on a block to insert it, it will convert this empty Paragraph Block to the new block that you selected.