SharePoint2016 On Prem -SPFx HelloWorld App deployed to App Catalog but not appearing under "Apps Yo

"Spfx webpart cannot work in Onprem as modern UI is not on it until 2019 server."

That's not actually correct. You can use SPFx web part in on-premises as long as you are running SharePoint 2016 Feature Pack 2. You do NOT need to use modern experiences or UI with SharePoint Framework web parts. 

1. Ensure that your server is running SharePoint 2016 Feature Pack 2 or newer
2. Chose 'onprem' as the option when creating a web part in the Yeoman flow

Upcoming SharePoint 2019 (preview starts soon) will also have modern UI, which then enables also SharePoint Framework extensions in the on-premises.