two ways to install zulip on android: play store or github
(there's also fdroid but its either unmaintained or just doesnt include push notificaiton support)
play store: https://play.google.com/store/apps/details?id=com.zulipmobile&hl=en_US&pli=1
github: https://github.com/zulip/zulip-flutter/releases/latest
for github, you can use an app manager like obtainium that will check for updates automatically
obtainium: https://obtainium.imranr.dev/
from there, “add app” → paste in the github repo link (https://github.com/zulip/zulip-flutter) and it should automatically figure out how to install the apk from there