Explanation

Box 1: No
Box 2: Yes
Box 3: Yes
You can create links to information and features within the Teams client. Examples of where this may be useful:
* Navigating the user to content within one of your app's tabs. For instance, your app may have a bot that sends messages notifying the user of an important activity. When the user taps on the notification, the deep link navigates to the tab so the user can view more details about the activity.
* Your app automates or simplifies certain user tasks, such as creating a chat or scheduling a meeting, by pre-populating the deep links with required parameters. This avoids the need for users to manually enter information.
Reference: https://docs.microsoft.com/en-us/microsoftteams/platform/concepts/build-and-test/deep-links

Explanation
You need to update the web part manifest to make it available for Microsoft Teams. Locate the manifest json file for the web part you want to make available to Teams and modify the supportedHosts properties to include
"TeamsTab".
Reference:
https://docs.microsoft.com/en-us/sharepoint/dev/spfx/web-parts/get-started/using-web-part-as-msteams-tab

clientState specifies the value of the clientState property sent by the service in each notification. The maximum length is 128 characters. The client can check that the notification came from the service by comparing the value of the clientState property sent with the subscription with the value of the clientState property received with each notification.
Note: A subscription allows a client app to receive notifications about changes to data in Microsoft Graph.