Hey Jan,
handling multiple user interactions is not as easy as it is in sap gui. SAPUI5 apps are stateless. You can't lock a business object (f. e. a sales order) when a user enters a app to change this object because the user is able to close the browser window without notifying the backend. The object will stay in lock mode.
To prevent loosing information when a other user changes an object during your work on this object you can use etags.
ETag Handling - SAP NetWeaver Gateway - SAP Library
In S/4hana SAP introduces the so called soft state. If a user enters an application to edit a business object, this object will be locked for a short period of time. If a user closes his browser, the object will be unlocked after a short time by the system. Furthermore users can work with drafts. A user can edit informations of an object and safe this changes as a draft. Later the draft can be edited again or can be activated so that the changes were written to the business objects on the database.
Greetings
Simon