Summary
SharePoint 2019 delivers an updated modern look and feel for lists and libraries and enabled by default.
Modern: Classic:
However, if the classic experience is required for your farm, the modern experience can be disabled.
How to modify the default experience for list and libraries as a user
From the user perspective, switching from modern and classic is a simple click.
For example, users will see 
all lists and libraries. Once in classic mode, you will then see 
to return to modern mode.
How to modify the default experience for list and libraries as an Admin
Admins can manage the default experience for list and libraries at the site collection, web or library level.
To change disable / re-enable the modern user experience at the site collection level
#Site Collection Level
Add-PSSnapin microsoft.sharepoint.powershell -ea 0
$site = Get-SPSite http://spwfe
#Disable modern Lists and libraies at the Site Collection Level
$featureguid = new-object System.Guid “E3540C7D-6BEA-403C-A224-1A12EAFEE4C4”
$site.Features.Add($featureguid, $true)
#Re-enable the moden expirence at the site collection Level.
$featureguid = new-object System.Guid “E3540C7D-6BEA-403C-A224-1A12EAFEE4C4”
$site.Features.Remove($featureguid, $true)
To change disable / re-enable the modern user experience at the web level
#Web Level
Add-PSSnapin microsoft.sharepoint.powershell -ea 0
$site = Get-SPWeb http://spwfe
#Disable modern Lists and libraies at the Web Level.
$featureguid = new-object System.Guid “52E14B6F-B1BB-4969-B89B-C4FAA56745EF”
$site.Features.Add($featureguid, $true)
#Re-enable the moden expirence at the Web Level
$featureguid = new-object System.Guid “52E14B6F-B1BB-4969-B89B-C4FAA56745EF”
$site.Features.Remove($featureguid, $true)
To change disable / re-enable the modern user experience at the library level
Add-PSSnapin microsoft.sharepoint.powershell -ea 0
$web = Get-SPWeb http://spwfe
$list = $web.Lists[“Documents”]
#Classic setting
$list.ListExperienceOptions = “ClassicExperience”
$list.Update()
#Modern setting
$list.ListExperienceOptions = “NewExperience”
$list.Update()
#User Default
$list.ListExperienceOptions = “Auto”
$list.Update()
More Information
I hope you found this tip useful, and if you are interested in the SPO equivalent, see the following articles.
Switch the default experience for lists or document libraries from new or classic