What is the best approach for using CiviCRM as an app on mobile?
The combination of the two extensions CiviMobile (thanks @rahulbile) and Summary fields is very interesting.

The CiviMobile extension is poorly documented, but after a while I figured out that it uses profiles to choose which fields to display. Also different profiles for each contact type: Individual, Household and Organizations. It took me more time before I figured out that it used the restricted profile_ids 4, 5 and 6, which can be edited to add more fields.

Then I could also use some of the fields from Summary fields, to display relevant contribution informations as search result on the contact.