How to get the objects of objects of array when the object key is not known?

You can use

array.forEach(obj => {
    let key = Object.keys(obj)[0];
    let yourValue = obj[key]; 
    // yourValue = { "age": "21", "id": "rcjcx8" ...} in your case for the first item
});

If you are fine with Lodash, you can do the following:

// assuming the above data is in variable "data"

let structuredData = _.flatten(data).reduce((a,c) => _.assign(c), {});

Iterate and get the keys and use the key to get the value. Working version

import { Component } from '@angular/core';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ]
})
export class AppComponent  {
  name = 'Angular 5';
  jsonDataKeys : any=[];

  jsonData : any =[{
    "-KzbaPS3vuZTG2LbuB3j": {
        "age": "24",
        "branchname": "columbia",
        "city": "hyd",
        "date": 1511418820987,
        "gender": "M",
        "id": "rcjcx8",
        "mobile": "9898989898",
        "name": "Patient1",
        "op": 100
    }
}, {
    "-KzTh7RTtTWviL4HkHv0": {
        "age": "21",
        "branchname": "kims",
        "city": "vizag",
        "date": 1511334303070,
        "gender": "F",
        "id": "45krxb",
        "mobile": "9898989898",
        "name": "aruna",
        "op": 100
    },
    "-KzTiCIheMPJIpJhPXQJ": {
        "age": "22",
        "branchname": "kims",
        "city": "hyderabad",
        "date": 1511420593865,
        "gender": "F",
        "id": "c7iqws",
        "mobile": "9878787878",
        "name": "vineesha",
        "op": 100
    },
    "-KzTnzLnRsdFWg-m5b9U": {
        "age": "26",
        "branchname": "kims",
        "city": "bheemavaram",
        "date": 1511420593865,
        "gender": "F",
        "id": "ujtgz",
        "mobile": "9876787898",
        "name": "Madhavi",
        "op": 100
    }
}];

constructor(){
  for(let arr of this.jsonData){
       Object.keys(arr) && Object.keys(arr).map((key)=>{
          console.log("push")
          this.jsonDataKeys.push(key);
        });
    }
}

}

HTML file

{{jsonDataKeys}}

<hello name="{{ name }}"></hello>
<p>
  Start editing to see some magic happen :)
</p>
<!--<div>{{jsonData | json}}</div>-->

 <div *ngFor="let arr of jsonData">
  <div *ngFor="let item of jsonDataKeys">
    item : {{item}}
    {{arr[item] | json}}
    <br>
    <br>
  </div>
 </div>