Computer
On iPhone use Safari. On Android use Chrome. On this computer, Chrome and Edge can install a windowed app.
This computer
Look for the install icon in the address bar, or the menu item Install Nativ. It opens in its own window.
Phone
Open it in Safari on iPhone, or Chrome on Android, then follow Add to Home Screen.
This is a web app you install to the Home Screen — the same path Instagram used before the App Store. No Apple or Google review. After you publish Nativ, send people this Install page.