Once your app is real, new work reaches it in one of two ways, depending on what changed.
Bigger changes — a new screen, a new permission, anything that touches how the app itself is built — go through the same review every app goes through: Apple's TestFlight for iPhone and iPad, and Google Play's internal testing track for Android. The first time, you accept a short invite from each store; after that, new builds simply appear as an update in the same place any app you already use gets its updates.
Smaller changes — wording, a fixed bug, a small visual tweak — do not need a new store review at all. They are sent straight to your app and are already there the next time it opens, with nothing for you to install.
A quality-checking pass taps through the real, running app first, on both Android and iPhone, checking that nothing broke. If that pass finds a problem, the release is held back rather than sent out broken.
You will get a notification when a release goes out, and the Releases page in your dashboard always shows what shipped, when, and what changed. If a release ever needs your say-so first — say, a change to something public-facing — it appears in "Needs You" before it goes any further.