SharePoint lists have an out-of-the-box setting that can be turned on to allow only users who create the item to view/edit the item. With SharePoint lists, you define the columns as the fields you want and it builds a basic form by default. You can customize this later using the 'Custom Form' option, which utilizes the Power Apps platform. The administrators of the site will have full access to view/edit the list items.
Now if you need one user to create and assign a task to another user for them to modify and complete... That requires a Power Automate solution, where you can grant/remove access to specified users on item creation.