You can only get sObject details if you add an Id = 'flowId'
to your query in the Tooling API or call the /tooling/flow/flowId
endpoint.
The info is in the Metadata of the flow which you can only pull when you get one record. You'd have to query all process builders (Process Type = 'Workflow') and then get the info for each specific process builder returned to get the sObject label with /tooling/flow/flowId
or, if you have the Ids, query with a Id = flowId
for each flow (or combine into one composite request).
I simplified what comes back in the "Metadata" portion of the response to only include the processMeatadataValues
you'd be interested in. It's under the "ObjectType" name
and contains the label in stringValue
"Metadata" : {
"processMetadataValues" : [ {
"name" : "ObjectType",
"value" : {
"booleanValue" : null,
"dateTimeValue" : null,
"dateValue" : null,
"elementReference" : null,
"numberValue" : null,
"stringValue" : "Account"
}
}],
}