The way that I learned this was through the usage of
User Controls, which will be a file pair (markup page + code behind); that can be called in a web-forms page.
There is some more work that needs to be done than using partials within the MVC paradigm, but as it is quite mature there is plenty of documentation and tutorials available for it
Some Tutorials:
ASP.NET Web Forms Tutorial: User Controls Examples[
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Creating User Control in ASP.Net[
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