ionic 4/5 app open from browser button click

You can use ionic's InAppBrowser for that. Here is the link to the plugin https://ionicframework.com/docs/native/in-app-browser

If you want to open within the app, you just need to pass the following parameter while opening,

  constructor(
   ...
    private iab: InAppBrowser
  ) {}

openInAppBrowser(){
  this.iab.create(`url`, '_self', {
        location: 'no'
      });
}

try this window.open(url, target, options);

ref: Reference to the InAppBrowser window. (InAppBrowser)

url: The URL to load (String). Call encodeURI() on this if the URL contains Unicode characters.

target: The target in which to load the URL, an optional parameter that defaults to _self. (String)

_self: Opens in the Cordova WebView if the URL is in the white list, otherwise it opens in the InAppBrowser. _blank: Opens in the InAppBrowser. _system: Opens in the system's web browser. options: Options for the InAppBrowser. Optional, defaulting to: location=yes. (String)

Just try this,

import { InAppBrowser } from '@ionic-native/in-app-browser/ngx'

constructor(private router: Router, private iab: InAppBrowser) { }

openUrl(){

this.iab.create('http://google.com', '_self', {
  location:'no'
});

}