What are the options I can provide a user for inputting data into a SharePoint List?

"Are Microsoft SharePoint Lists intended for users to use the same standard User Interface that the developer uses to add rows to the list?" Yes. But then you don't need to be a developer to create a list.

"If so, are users able to do this on their cell phone?" Yes, there is a Microsoft Lists app and users can enter and edit data in a list on it.

 "Are there extensions to SharePoint that can allow an improved UI/UX for entering a row of data in into a list in a way where that is the only task offered to the user?" No. Using just a list a user would be able to see all the items. You could however build a simple app in Power Apps connected to your SharePoint list, add a form and set the default mode to New. Then add this app into your SharePoint page with the Power Apps web part as shown below on the right: 

...or open it on a a cellphone using the Microsoft Power Apps app. This is how the app from the image above looks on my cellphone:

Rob
Los Gallardos
Microsoft Power Automate Community Super User.
Principal Consultant, SharePoint and Power Platform WSP Global (and classic 1967 Morris Traveller driver)