What do I need to study?

Question

Android

Question from hardened 1Snick. But in the last century, I remember, I had a relationship with one of the varieties of Basic.
Help put the puzzle together. Unfortunately, you can't do it on your own. There is too much information and I'm not entirely sure I'm drawing the right conclusions from this thread.
I want to write an application. Let's say an analogue of evernote. A notes database that should work on mobile devices and on PC as an application or in a browser.
So far, I have not come across an intel that is understandable to me on the net about how such applications are built. As far as I understand, there should be a server part that stores data and synchronizes it between mobile devices apparently using web services. This is one part. There must also be a web interface to work through the browser directly with the database. And there should also be a mobile application that also stores some of the information on its own and periodically exchanges with the server part.
Am I correct or is there a simpler way? Maybe they have already come up with some kind of framework in which one application is written, but then it can be compiled for different platforms? In general, in 1C you can develop all these parts in one config. One instance will work as a server part and a web interface, and for mobile devices it will also build an application from the same config. But I want to leave 1C.
What do I need to learn to implement this?

Answer 1

0. In most cases, you need one thing or web, or mobile phones, or desktop
1. If you don’t understand how to do everything as a whole, do what you understand on the platform that you know
2. There are "frameworks" that implement this, but they have their limitations, which are often not
acceptable

Answer 2

How to invent nextcloud if it has already been invented.

Answer 3

If you want to start learning Android development, then here is an algorithm for how to write a simple version of the application that will store records locally in the database. This, of course, is much simpler than your idea, but it will introduce you to the topic:
1) Learn Java Core (syntax, oops, Collections, Generics) + Android SDK (Activity, interface layouts, RecyclerView, Intents UI, Worker Thread, SQLite). At the first stage, the Head First book for Android + docks from Google is well suited.
2) Learn what the abbreviations MVP MVVM mean
3) Once you understand the basics, take a look at Google's codelabs site for Architecture Components (a set of architecture libraries along with ORMs that provide higher-level tools for building applications). There are examples of simple cruds that work with a local database
3) In the future, to create client-server applications, I recommend studying the Retrofit library (this is an http client) + Json, XML parsers (GSON, XMLViewer)

Answer 4

https://ru.wikipedia.org/wiki/%D0%9A%D0%BB%D0%B8%D...
https://ru.wikipedia.org/wiki/HTTP
https://ru.wikipedia. org/wiki/REST
https://ru.wikipedia.org/wiki/%D0%90%D1%83%D1%82%D...
And so on.