In the past they were indeed the same but nowadays there is no straightforward way of doing this. The MDA itself is just a zip file, but it contains a folder called 'model' in which there is a model.mdp which is a binary file of your application model. With a lot of effort you may be able to extract some information from it. But you definitely can't reverse it into an mpr, if that's what you're after.
If it's JUST the domain model you're after and you can still run your application, you could try logging in with a role that has access to all entities. The client will then receive information about these entities when loading the application in your browser. If you snoop the network traffic or if you set the JSON lognode on the server to debug, you can check this information.
I would turn my hopes towards finding a repository in which you still have the actual model instead.
An .mpk file is is just a zip file with a different extension. On my laptop, double-clicking an mpk file will extract it from the archive. If this does not happen for you, you can change the file extension to .zip and then extract the contents.
The extracted contents should contain a project .mpr file which you can open in the Modeler.
.mda file is compiled file of mendix code and we deploy it to cloud. We cannot go reverse means we cannot convert compiled code into mendix code to open it in mendix studio pro. With limited experience I am putting this.