A search for boys AND China or an "All the Words" search for boys China will only give you results containing BOTH words.
Use AND or All the words searches to narrow or focus your results.
A search for heart OR cardio* or an "Any of the Words" search for heart cardio* will give you results containing EITHER word.
Use OR or Any of the Words for synonyms or closely related words.
Remember that scholarly writing (academic books and peer-reviewed journal articles) takes a longer time to produce than journalistic writing.
If your topic is a fairly recent phenomenon, or if it hasn't been noticed yet, you may not find much in the scholarly literature.
(Chinese OR Korean OR Japanese)
(cuisine OR cooking OR recipes)
...will give you articles/books on the food of any of these countries.
Quotation marks force phrases:
Asterisk finds word variants starting with a string of letters:
Google Books allows you to use the asterisk to subsitute for words in between other words: