A recent app I developed in NextJS is ContaDocs - built to help accountants. They only need to register in the dashboard with their Google account to allow ContaDocs App some Google Drive permissions and add the companies they are doing accounting for. For every added company, they have to set the phone number of the representative of that company.
Now, every company representative can log in to the app with a phone number and a One Time Password, and they can upload monthly files like invoices or bank account statements until some deadline day configured by the accountant above.
This way, the accountant will have all the files updated.