Hi Ramesba,

1) I'm not entirely sure about question 1, I don't think I entirely understand the question, sorry.

2) Yes, this can be done, and there's a nice starter Trailhead available (https://trailhead.salesforce.com/content/learn/modules/mobile_sdk_react_native).

3) This answer to this is the common "it depends"... and this is based upon your requirements, budget, skill set etc.
For example you need to answer questions like the following, and this will help you decide on the initialr route to take (e.g. custom or not)

• Do you really need a mobile app, or will the mobile web experience be enough
• Who are the users of the app? What licenses will they use? Are they employees or customers? If they are employees then perphaps using the mobile Salesforce app should be the route (to share logic between desktop/mobile) If the app is for customers does the app need to be on iTunes/Google play, and does it need to be branded (if so then a custom app is needed - perhaps Mobile Publisher could be used)?
• Does the app need to access harward APIs on the device (e.g camera, location)? If yes then a custom mobile app is required.
• What devices is it likely to run on? The Salesforce products have relatively small numbers of older OS that are supported.

It is likely that since you're asking about React Native that you've already decided that a custom mobile app is required, but I thought it best to go through the above just in case.

In my opinion using something like React Native gives a lot of benefits, including support with the Salesforce Mobile SDK and the single-code base for multiple devices. Other alternatives would be to use the SDK and frameworks like Ionic with Angular/React/Vue... I'm a big fan of Ionic.

When building almost any custom app it's worth keeping in mind the following too;

• The ecosystem is constantly moving, and having a process to ensure the apps keep running through the releases of Salesforce, iOS, Android, etc, is something the should not be truly considered as we've seen updates to all of these pieces cause issues for apps.
• Be aware of API calls to the platform, each interaction will count against your quota
• Offline support is hard, but in many use cases the support for offline running is absolutely critical
• Using Salesforce Mobile SDK has it's own drawbacks. There is no offical support channel, and in my experience I wouldn't jump straigh to the latest release (for stability reasons)

As for alternatives I have already mentioned other hybrid-type tech such as Ionic, but you could also build natively. This option might be for you if you have a sinlge OS and/or the native skills sets (swift/Java). IMO the performance difference these days between hybrid/native is so small it's negligible, and if an app is built well then the users will have no idea if it's native/hybrid.

At my company (MobileCaddy) (https://mobilecaddy.net) we have a app-dev framework that takes care of the consideration points above, and allows the devs to focus the business logic. We support 100% offline running, with full custom UI and have zero impact on the API call quota. Our partners (and direct customers) build their apps using Ionic so also get iOS/Android support, as well as windows desktop too.

Apologies for the long reply... but as I said there is a lot to consider when wanting to mobilise Salesforce.