Text Box above SharePoint List

Yes, in classic mode you could use Edit page, and insert web parts above the list web part. Microsoft removed that functionality in modern view pages. The supported method would be using a new page and then adding text above and and displaying the list below. If you want to modify the New/Edit/Display forms then using PowerApps is your best bet. 

You can read more about it at https://docs.microsoft.com/en-us/sharepoint/dev/solution-guidance/modern-experience-customizations-customize-lists-and-libraries.