See thesolution in the explanation below
Explanation:
To add a disclaimer to all Teams meeting invites, you can use the Exchange admin center to create a mail flow rule that applies the disclaimer to meeting requests. Here are the steps to follow:
Sign in to the Exchange admin center with your Microsoft 365 username and password1.
In the left navigation, go to Mail flow > Rules.
Click Add (+) and then select Create a new rule.
In the New rule pane, enter a name for the rule, such as "Meeting disclaimer".
Under Apply this rule if, select A message header includes any of these words.
In the Specify header name box, enter X-MS-Exchange-Calendar-Meeting-Request. This is a special header that identifies meeting requests2.
In the Specify words or phrases box, enter True. This means that the rule will apply to any message that has the header set to True.
Click Add and then OK.
Under Do the following, select Append the disclaimer.
In the Specify disclaimer text box, enter the disclaimer text that you want to add to the meeting invites, such as:
Disclaimer: This meeting may be recorded for quality and training purposes.
Click Enter text and then OK.
Under Choose a mode for this rule, select Enforce.
Click Save.
Now, when you or anyone in your organization schedules a Teams meeting, the disclaimer will be added to the meeting invites automatically.

Explanation:

Reference:
https://docs.microsoft.com/en-us/microsoftteams/microsoft-teams-online-call-flows

Explanation

Reference:
https://docs.microsoft.com/en-us/powershell/module/skype/grant-csteamsapppermissionpolicy?view=skype-ps

See the solution in the explanation below
Explanation:
To prevent users from deleting messages posted in private chats and channels, you need to manage the messaging policies in Teams. Here are the steps to follow:
Sign in to the Teams admin center with your admin credentials.
Go to Teams apps > Messaging policies.
Select the policy that you want to modify, such as the global (Org-wide default) policy or a custom policy.
Turn off the following settings: Owners can delete sent messages, Users can delete sent messages, and Users can edit sent messages.
Click Save to apply the changes.