Welcome! 
SmartThings allows customers to upload ācustom codeā into their accounts to extend what is available beyond the official features. This is great. 
This also wonāt change the mobile app you use. You will still use the regular SmartThings mobile app. It just adds an additional feature which you will be able to access from inside the SmartThings mobile app.
Even better, there are many community members who write custom code for themselves and then are happy to share it with other community members, sometimes for a small fee, but usually for free. The process of using someone elseās code is pretty straightforward: basically you copy it from the author and then you paste it into your own account.
The following is the FAQ on how that process works (this is a clickable link)
So if you read that and it sounds like something youād be interested in doing, then how do you find the custom code to copy? Most of it is in github and is linked to from an authorās thread in this forum, but searching this forum just is not easy.
Fortunately, there is an easy way to find a custom code. You can use the quick browse lists in the community ā created wiki. This is organized into three sections: smart apps, device type handlers, and project reports. Within each section, the lists are identified by topic, such as the one you will be looking for, which is a smart app for locks.
When you bring up the list, there will be maybe a dozen different threads which will describe different applicable code or projects. You can read about it in that thread and you can also ask any questions about that specific one in that thread and people will be glad to help you.
So this makes the whole process much easier. You might also want to take a look at the āget startedā list in the project reports section, as it has a lot of welcome topics and things like ātop 10 things to do with SmartThingsā and āwhat device should I get next?ā
( Also, I should say that the specific smartapp you are probably looking for is just called ālock manager.ā Itās free and very popular. Again you will find it on the ālocksā quick browse list in the smart app section. )