Answer– You will want to use your custom domain and you can use subdomains as you’d like, however, you will need to create DNS records for each subdomain in your DNS host.
Answer– Yes, you can create any subdomain you like, however, as previously stated you’ll need to add DNS records for each subdomain added in Mailgun.
3.) Currently, I have a test MailGun Account. So my question is that API key will be the same for all domains or it will be changed?
Answer– The API Key will be account wide, however, the SMTP credentials will be per domain.
4.) If I will create a new Domain by MailGun API that time MailGun will create a default login SMTP Credentials or I need to create by MailGun API?
Answer– If you add a domain using Mailguns API then SMTP credentials will be applied when the domain is applied.