They will not have email servers for each client progrem, but I suspect that you are talking about three different services there. On only one of them is your ISP acting as an email server. For the other two, your ISP is just providing connectivity to the internet, with the email servers being hosted by other organisations (goolgle for GMail and Microsoft for outlook.com).
Thunderbird is an email client program and may well be pointed at the ISPs email server, to collect email from the email account that comes bundled with the connection.
ISPs will frequently have many servers forming their email infrastructure, but apart from having different functions in the grand scheme of things, they the only split from a customer PoV will be between home user and business email.
ps "Outlook" could also refer to an email client program that could collect email from your ISP email account. I was assuming that your use of Outlook refered to Outlook.com (formerly Hotmail) - the web based free email provider.