My application needs to log in to each user and generate the bearer token for each of them. By using the bearer token I can fetch the result using Twitter API v2. Anyone, please help me how can I do that.
The bearer token is specific to your application, NOT to the user. Bearer token / app only authentication is JUST for the app, not for the user. The first time you create the app on the Twitter developer portal, you get the bearer token which you should save in your password manager.
The account token - when a user is logged in to your app - is different. If you need this, you can also generate it on the developer portal FOR YOUR ACCOUNT ONLY, or you can make your app implement sign-in with Twitter via OAuth 1.0A. These tokens will enable your app to operate for a user and port new Tweets, like Tweets, create lists, etc.