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
app/design/frontend/base/default/template/page/html/breadcrumbs.phtml
Being added into
app/design/frontend/argento/<my_theme>/template/page/html/breadcrumbs.phtml
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
vendor/magento/module-theme/view/frontend/templates/html/breadcrumbs.phtml
A slider file is here
app/code/Swissup/EasySlide/view/frontend/templates/slider.phtml
Where would these need to be placed to display using my own theme, which is below
/app/design/frontend/Local/<my_theme>
