Hello @ALalla917 [sorry for the lengthy response]
The last bullet point is directly dependent on the bullet points 1 and 2
Yes you have to take action only in the Orchestrator, but the name has to be the same as what you have in the config.xlsx
GetCredential is the activity that we use to get the details of the credential type asset that we create in the Orchestartor. Detailed description on the activity is in the link below:
whereas, GetAppCredential - this is one of the sequences what has been created as part of the Robotic Enterprise Framework that has been created following best practices for business processes.
Actually the config.xlsx is like a common property or settings file(if i speak in development terms)
It allows you to list and use the common settings or configurations(that is why the nomenclature) across the project. In this case the REFramework.
In simple processes/automation where you are not doing much you may not need to create one, but in terms of business process you have a lot of configuration example- URL, Credentials, queues.
In the REFramework, which UiPath is trying to make you familiar with during the assignments, this config file which is of xlsx type is used.
So In the InItAllSettings.xaml, which you can find under the Framework folder, the config.xlsx is read and all the data is stored in a Dictionary of Key and Values. like URL(key) - https://acme-test.com(value)
This you use through out your assignment/project to get the values.
Hence the implication. What you add in the sheet as key and value, you can use that in your process by providing the key and it will get you the value. Example below:
You have a key System1_Credential and value System1Cred then the asset name in Orchestrator has to be System1Cred 
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