There is no "Thunderbird server", so that resolves the first part of your question. I'm not sure what you mean by "log in". Thunderbird, by and large, fetches your messages from the server when you tell it, or you can set it to do so periodically. To get your messages, it will need to know the email account's username (very often all or part of your email address) and the corresponding password. If you allow Thunderbird to store the password, you don't even need to think about "logging in" to the mail server. I said "by and large" because with some IMAP servers, messages appear to be fed to Thunderbird as and when they become available, assuming that it is alive and on-line.