While this might be apparent to many, here is a brute force method to search a drupal 9 for the Body field for any matches of a string. I am not a Drupal expert, so finding the table + field was a bit of a chore.

SELECT * FROM node__body WHERE body_value LIKE ‘%string%’;;

The SQL query selects all columns (*) from the body_value inside node__body table where the post code lives. The % symbol is a wildcard operator in SQL and is used to match any number of characters before or after a specified pattern. Be sure to surround your query with the %. For example, if searching for "Hello World!", it would be %Hello World!%. You can search for HTML within the individual fields as well.

Of course this can easily search multiple entires by using AND. 

SELECT * FROM node__body WHERE body_value LIKE ‘%string%’ AND body_value LIKE ‘%string2%’;;

Using a GUI like Sequel Ace, you should be to see which bundle it's in, if you've set up different page types, and the entry_id which can, of course, be accessed via PHP or simply by going to sitename.com/node/entry_id or sitename.com/node/entry_id/edit to get directly to the admin "edit" panel for the node.