To download models, textures and other assets at runtime from your server in Unity, you should use Asset Bundles. You can read about them in the Unity Documentation.
The basic workflow for them is that you take your assets and import them in Unity and then generate an asset bundle from them. You can also split your models into multiple asset bundles. You then export from Unity and upload them to your server. Downloading them back is done through APIs provided by Unity, which you can find in the documentation linked above.
All user interaction, as well as deciding what models to show and what textures they can have, can also be done in Unity, without writing any iOS or Android specific code.
I did not fully understand the first part however. Do you already have an existing iOS / Android project and you want to integrate Unity on top of that?
thank you for this answer, now I'm going to study assets bundles.
Regarding the first part I try to rewrite it:
Usually we create apps with a view for RA and other view for other stuff, on application's start we download all necessary files, can I manage the downlod with osx/android code?
Can I downlod the bundles, put them in a folder (for example in iOS in document folder) and then get them from unity?
Or I need to download this bundles with unity code? (seems to be a stupid question but I'm very new with Unity and it is a new world)
You could manage this manually in iOS / Android code, because Asset Bundles are just files in the end. After you download them you still need to load the asset bundles in Unity so that it can decompress and process them.
But Unity could also do the downloading for you, including caching the asset bundles using WWW.LoadFromCacheOrDownload for example. In the end it's up to you which method you prefer, but if you want to use native iOS or Android code with Unity, you will also need to familiarise yourself with how Native Plugins work in Unity, so that you can communicate between C# and native.
I'm new of Wikitude/Unity and I have a question about the integration between a Unity project with Both Android / iOS project ... I need to manage my model through an online backend, so at the start of application I need to download some model and then, Depending on the button clicked I need to change texture or change / add models.
My question is:
all of this actions, must be done in Unity or iOS / Android?
after download my models there is a way to use the models on a Unity scenes?