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can the app be downloaded only from Google Play and the Apple Store?
Yes you can do that. But in my case the whole thing with Apple took over half a day to sort, any you’ll need at least an Apple PC/ Laptop and a iPhone or iPad
Is it also possible to download and install an app from a website?
I don’t think thats possible
That means, whenever someone has installed the app… I can send them push notifications, SMS messages and emails?
For push notifications yes. For SMS an emails, the app can send these regardless of whether that person is an app user or not. The App just needs to have their SMS number and/or email address
And I don’t need any third-party connection, plugin, provider, call it what you want?
Push notifications = No. The App can do that itself
Emails = No. The App can do that itself
SMS = Yes. Appsheet has an integration with Twilio. But I’ve found that SMSBroadcast.co.uk works great via a workflow
Is there already a template I could use, just for that purpose?
I doubt there is one thats perfect for what you need. My suggestion for Appsheet newbs is always to get an Appsheet Dev to do 2-3 hours of workng getting a app roughed out. Then you can pick through what they’ve done to understand it. You can then start making minor mods yourself. I can do this for you if you want. But in the intersts of fairness also checkout @MultiTech_Visions and @Bellave_Jayaram who are both good too.
Is there a web-based user interface I can use to send messages?
When you create an App it give you the option to use is via a Google App, Apple App or browser interface
Hope this all helps 
