Theoretically, yes, may be? practically? not really. If you do not consider people’s time, then apps can even be free. How much time does it take to build an app? and that itself is a decent cost. Compare to those,
AWS offers one-year free hosting. Digital Ocean $5 virtual server is almost equal to AWS nano server, which is $0.0059 per hour ($4.24 per month). When it comes to scaling, you need more servers, add a load balancer, and suddenly price goes up. You can make Google Docs/Sheets as your DB, but when you have millions of users, you will millions of rows of data as well. Good luck search/manage from these. oh, and communicating with those millions of users (assuming, your system sends one email per user per month), who is going to give that email delivery for free?
I think these tips are good for MVP, but not for that volume. When it comes to millions of users, then there won’t be many free options, but that is a good problem to have.