The work made specifically for you is yours once the engagement is paid for. We keep the reusable components and methods we bring to every project, and you get a permanent licence to keep using those as they are built into your site. That split is in the master services agreement rather than being a promise made in an email.
We will also stay on for a handover period if you want one, and remove our own access when you say so. Nothing about the arrangement depends on you being unable to leave.