I am looking for a way to add a text box (image attached – https://ibb.co/JpWTCY7) in most of my posts.
It will be a good idea if I could add them once by category using a plugin than inserting the code in every post.
Block element allows you to place a Layout created within Gutenberg editor which you can hook to certain parts of the page. You can also specify with pages/post you want it to appear.