I would like to know how different apps like Sales, Marketing, Service etc are chosen from the Salesforce mobile app.
The Salesforce mobile app doesn't allow the user to navigate between different Apps (tab sets) as is done in a desktop browser. You can't even assign different page/tab configurations for different groups of users currently. You modify a single set of available navigation items for all mobile users in the org through the Salesforce Navigation item under Mobile in Setup. Users' permissions are applied, though, so you can add items to the navigation and they will only be visible to the users who have been granted access to that item.