SharePoint is providing a role on each server that is called "Microsoft SharePoint Foundation Web Application". if you check the "services on server" option in central administration you can see which server are currently configured to have the iis sites created.
To unjoin the SERVER1 from the farm you have to update the alternate access mapping for the web application to point to the new server. You have to validate the needed role is started on the new server. Than you have to tell all users the url has changed. Under all circumstanses i would try to avoid such an operation.
I would recommend creating the web application in the first place always with a host header information and create a DNS entry to the server (or servers if some kind of load balancing is used). There are some additional todo's if kerberos is used, but in general not using the servername is a best practice.