I am using sharepoint on premise server 2016.I want to open files in online mode(browser) but while opening all files are getting downloaded.I want to implement this feature to edit documents online.I haven't activated 'Open Documents in Client Applications by Default' feature too.But still it is opening in my local application.How to open files in browser mode to edit it?
In order to open office docs in the browser you must have an office online server 2016. Can you verify that it is installed on your farm? https://docs.microsoft.com/en-us/officeonlineserver/deploy-office-online-server
I agree with Kasper, you need to install Office Online Server(OOS) for SharePoint 2016 to open office files in the browser. You could refer to this article to configure OOS for SharePoint 2016: