In Magento 1 if I wanted to modify my own version of a phtml file I would move it into the theme directory, while maintaining the same structure. I have needed to do this on a single Magento instance that has multiple store views and uses different themes.
An example would be
Being added into
When viewing the website, my updated template would be used and not affect any other store views I have. This method didn't require any xml updates, or technical wizardry, which suited me perfectly.
Magento 2 seems to be structure quite differently The breadcrumbs file is held within
A slider file is here
Where would these need to be placed to display using my own theme, which is below