Move SharePoint 2016 on prem environment to Windows 2019 and SQL 2019

Hi Jupiter82 ,

Have you named the site url in the new (win2019) environment same as the existing one (win2012r2) ?

If you are trying to reach the SharePoint site locally in the server you need 1st to add an entry into the hostfile, 2nd  Disable Loopback Check via PowerShell and 3rd (optional) configure the SSL certificate in IIS if your site is running in HTTPS.

This should help you accessing the site in the new platform (win2019).

Concerning the migration approach, what you are trying to do is technically possible because you are still using the same SharePoint version, only upgrading to a new OS, HOWEVER I don't recommend the approach because it's too risky, in my opinion, and if something goes wrong you will generate an outage.

A better approach is to build a parallel SP platform (as you are doing right now).

Apply all the settings and customization and migrate finally the Content Databases from an instance to another.

IF you need to access both platform at the same time, you need to create the web applications and urls same as the existing AND temporary extend on the new platform the web applications with a unique name.

This will allow you to keep browsing both sites at the same time (eg. xyz.contoso.com - win2012r2 / xyz-2019.contoso.com - win2019). Once you have validated that everything works fine, before the final cut over, you can simply delete the extension and keep the original url as default