What you could build
A mobile app ships if/when Apple or Google says so
Building the app is the easy part. Getting Apple or Google to approve it is the slow part.
Website or web app
Apple App Store
Google Play Store
Cost
Free, or around $20 a month for hosting.
$99 a year, for as long as you want to stay listed.
$25, paid once.
Timing
Live the moment you publish it.
~24 to 72 hours per submission, and longer if something gets flagged.
~14 days of closed testing before your first release.
Review
Nobody's. You decide when it goes out.
Every version, by a person, against published guidelines.
Every version, by a person.
Notes
Needs to be responsive, meaning the layout adapts to a phone screen rather than shrinking a desktop page down.
First submissions are often rejected, and reviewers aren't always consistent. Two similar apps can get different answers, sometimes with a vague reason.
An individual needs 12 testers for those 14 days. A registered company skips it, but verification can take weeks.