How does a modern app work that works both on the web and on iOS/Android at the same time?

Question

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Guys, a serious question is ripe. I deal exclusively with front-end development and am not aware of these matters.
How do applications that work both on the web and natively on iOS/Android interact? For example, instagram or facebook. Do they just share the back end, or just the database, and then comes the implementation, which depends solely on the platform? The functionality is the same, data synchronization is also there, then, in my understanding, the iOS application simply implements the front part using native tools from the developer kit, and then simply steals data from the database, which lies somewhere on the hosting server. So? Thank you.

Answer 1

there are a lot of nuances regarding the fact that the application can work offline, for example,
make many requests to the backend at the same time
layout for screens also differs significantly
energy efficiency
memory consumption (hello memory leaks) animation
caching performance lifecycle fragmentation of devices security "just robs data" - this is working with api / network requests have a bunch of their own implementations and architectural differences for mobile devices, in other words, mobile native development has a bunch of its own specific nuances that are not taken into account when developing front-end / web apps

Answer 2

It depends.
In the simplest case, a common back-end for everyone, with a RESTful API (i.e., a database, say, in MySQL, and HTTP request handlers in PHP, a vivid example is the VK API), and the web face works through it, and official mob. applications, and third party applications can work.
Sometimes they do it differently, here VK has one back-end for the web muzzle (undocumented, not RESTful), for third-party applications - another (RESTful API with OAuth), but about official mobs. applications - I don't know, probably also a RESTful API. Google, Twitter - also about the same.
Sometimes protocols other than HTTP are used.
Usually these are instant messengers, chats (protocols - TCP, XMPP, sometimes some kind of protocols).
But what mob. applications work on such a protocol, does not mean the absence of a RESTful API, and web muzzles based on this API.
An example is Skype, it has its own protocol, but there is web.skype.com - a web face with a closed, but HTTP back-end.
I recommend taking a Fiddler sniffer and seeing for yourself what is where and how.