Please help me to translate default Wordpress theme strings...

Hi, I am translating a wordpress website but how can I translate the defaults fields that belong to the theme not set by the webmaster? I don't find any edit box for these!
For example there is a "Search box", and I don't find any field for it?
And also there is an "Upload box section" and "Select the file to upload", these strings are automatically written by the theme, how can I change the Wordpress theme language to change all these strings at once?

I am free right now, if you provide me more details I will help you. I will need to access wp admin and want to see what exactly want to translate.

Website is not mine and I am nto authorized.
I just need to edit strings that have not been added by the webmaster but these are default theme/plugin strings...

This tutorial is a bit old but I guess it still works:
https://www.ostraining.com/blog/wordpress/language-strings-wordpress/

I hope this helps.