replace source codes of a page

Hi Angiela,

You have two options:

1. Make Duplicate (screenshot) of page and translate that "clone" to 2nd language, this is the most popular method, used also for Articles. You can also use Export & Import feature. Then inside that new cloned page go to Page Settings > Basic > Language and choose 2nd language from the list.

2. Or you can buy & install FaLang Pro that allows you to translate content inside that same page. More info on the developer site.

Of course as 0 step those extra languages (ES. IT. DE. etc) must be installed first for Joomla itself.
Sometimes you have to also translate some front-end language phrases from SPPB.

How can I find the source codes of a whole page and then replace them?

In general in the database (table prefix_sppagebuilder) but you're a Joomla beginner, using that method to duplicate & translate content will be too difficult for you! So, I suggests using the method that Joomla offers from the Admin area (1,2).