Unfortunately, it is no longer possible to download the SalesforceA App, as it has been discontinued by Salesforce.
The SalesforceA app was very popular with many Salesforce Administrators as it made it very quick and easy to perform some of the main administrative tasks on your mobile device.
But, as of June 2022, SalesforceA has been removed from both the Google Play Store and Apple App Store.
If you were lucky enough to download it to your mobile device prior to this date, then you will still be able to use it, until such time as you delete it yourself.
RELATED RESOURCE: Salesforce User Management
It is not being developed or supported any further by Salesforce, so even though you have it on your phone, you may find it increasingly unusable as it becomes incompatible with their other platform updates.