Set Global Search Center in SharePoint 2016

So, you have your search center site ready for SharePoint 2013 (Use this article: Create Enterprise Search Center using PowerShell) as a next step, you should configure global search center from SharePoint central admin site to set default search center site for all site collections.

SharePoint 2016 default search center
Once the search center site collection is created, navigate:

• Central Administration >> Application Management >> Service Applications
• Click on the Search Service Application >> In Search Administration page, Click on "Set a Search Center URL" link
• Enter the URL of your Enterprise Search Center Site collection that you created and add "/pages" at the end. In my case, its "http://searchcenter.crescent.com/pages"

By this, we've instructed the SharePoint Search Service Application to use this site to submit search queries and view search results.

Change Global Search Center URL with PowerShell
You can also change the search center URL by using PowerShell

#Get the Search Service Application
$SSA = Get-SPEnterpriseSearchServiceApplication

#Set global search center
$SSA.SearchCenterUrl = "http://Search-Center-URL/pages"

$SSA.Update() 

Here the "Search-Center-URL" is the URL to your Site Collection created with Enterprise Search Center template.