The Dynamic Content available to you is decided by the the triggers and actions previous to the current step you are on,the defined variables for that specific entry box.
Your Flow only contains When a file is created in a folder and Start and wait for an approval (V2), which have limited definitions. If you want more definitions, you'll need to add more actions before the condition.
If you notice, when you select the To box, your items available are different than the Subject or Body fields, that is because the content is Dynamic, meaning it only shows you what you can use based upon what you have selected.
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