override templates(phtml) by custom theme
if you want to override
vendor\magento\module-catalog\view\frontend\templates\product\some.phtml first you need to create your custom theme, please refer this link for how to create custom theme.
after that create phtml file in your custom theme to override default one
thats it finally clear the cache of magento as well as browser, then check your page, new phtml file only reflected.
same way you can do for all templates override.