Hello,
You need to read this article. Shopware take html page from cache for pages, which described in this article.
Can you describe what exactly you want to do?
Some pages aren’t cached at all like account or checkout.
For example you can check:
engine/Library/Enlight/View/Default.php::222
assign function contain third parameter where you can set flag - should/shouldn’t variable be cached.
Also you can create frontend widget. Widget will be handle separately.