Angular 8 to 9 :-

1. Follow the official migration guide.
2. Add "ng add @angular/localize" and add "import '@angular/localize/init';" in polyfills.ts, remove entryComponents

Angular 9 to 11 :-

1. Add a slash before assets url (if you get - Module Error (Emitted value instead of an instance of Error)) - https://stackoverflow.com/questions/53024995/module-error-emitted-value-instead-of-an-instance-of-error

2. Hammerjs - https://indepth.dev/posts/1075/exciting-times-ahead-be-ready-for-angular-9#hammerjs-is-now-optional & https://stackoverflow.com/questions/63613272/hammer-gesture-config-import-error-after-updating-angular-project-from-7-to-10 & https://github.com/angular/components/blob/master/guides/v9-hammerjs-migration.md#the-migration-reported-ambiguous-usage-what-should-i-do