I have a SharePoint list with a choice field column; when a specific choice value is selected, a new section with new columns need to appear. Is this possible?
In SharePoint online modern experience, you can use conditional formulas to show or hide columns in a list or library form based on other column values.
=if([$Category] == 'Product Management', 'true', 'false')
[$Category] is in format
[$InternalNameOfColumn], which means you have to use the "internal name" of choice column in formula. You can get the internal name of your column by following this article: How to find the Internal name of columns in SharePoint Online?
Note: You have to add conditional formulas on individual columns in list form as you cannot write conditional formulas on sections in "body" layout.
Microsoft official documentation: Show or hide columns in a list or library form
Update from comments:
For advanced list form customizations, you have below two options: