Check out example codes for "wordpress search in custom fields". It will help you in understanding the concepts better.

Code Example 1

If someone is looking for a solution to this problem is to recommend the plugin:
https://wordpress.org/plugins/acf-better-search/

Code Example 2

So, I did another search today and this was worked first time. http://adambalee.com/search-wordpress-by-custom-fields-without-a-plugin/

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