As already mentioned you need to run npx create-sitecore-jss nextjs.

This is a new feature that was added to Sitecore JSS 20.0.0: https://github.com/Sitecore/jss/releases/tag/v20.0.0

New Features & Improvements [create-sitecore-jss]

• New package containing initializers / templates for JSS sample applications (#881), (#883), (#882), (#880), (#879), (#878), (#876)
• This replaces previous jss create method to create new JSS applications
• You can now create an application with guided prompts using initializer command of choice (npx create-sitecore-jss, npm init sitecore-jss, yarn create sitecore-jss) 🎉🎉🎉

So that is why jss create test-app nextjs works when trying to use Sitecore JSS 19.0.0.

As already mentioned you need to run npx create-sitecore-jss nextjs.

This is a new feature that was added to Sitecore JSS 20.0.0: https://github.com/Sitecore/jss/releases/tag/v20.0.0

New Features & Improvements [create-sitecore-jss]

• New package containing initializers / templates for JSS sample applications (#881), (#883), (#882), (#880), (#879), (#878), (#876)
• This replaces previous jss create method to create new JSS applications
• You can now create an application with guided prompts using initializer command of choice (npx create-sitecore-jss, npm init sitecore-jss, yarn create sitecore-jss) 🎉🎉🎉

So that is why jss create test-app nextjs works when trying to use Sitecore JSS 19.0.0. It doesn't work anymore with Sitecore JSS 20.0.0.