AltaMail has many options for the email signatures that appear in the emails you send. You can set a default signature for all accounts, a different signature for each account or choose a signature when you send an email.

You can choose a signature for a new email using the action menu (circle with 3 dots). Select 'Email Signature' from the action menu and then you can choose and edit signatures.
You can also choose signatures when you tap inside the signature area of an email to show the settings button. You can then tap on the settings (cog) button to view email signatures.

Signatures can be edited with custom fonts, styles, colors and embedded images. Advanced users can even edit the signature formatting code directly. More

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.

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