You can choose edit the default app signature in the app settings (View) - Email section - "Default Email Signature".
This will show the Email Signatures screen where you can select a signature. Tap on the "+" button to create and edit a new signature.

For each account, open the edit account settings (View) screen and then tap on the "Appearance" section. Then you can tap on "Email Signature" to select a signature for this account. Tap on the "None" signature to have no signature for the account and it will then use the app default signature.

When sending an email, if the app or account signature are not suitable, you can then use the action menu button in the top of the send email screen (tap on "...." if it is not visible). Choose "Email Signature" from the action menu and then you can choose a signature to use for this email.

You can add multiple email address aliases to each email account View. Each alias can have its own signature that you can select.