Testament talking a bit about how they wrote and recorded Titans of Creation.
Apparently, they work remotely! All of the members are a minimum of two hours away from each other. They email each other wav files and dry guitar signal parts.
They got together in the studio for about a month, it sounds like. There, the vocals were written. The instrumental parts are rerecorded.
It's an interesting counterpoint to new school band Blood Incantation's way of working in which they rehearse together regularly and record everything live. Of course, part of why this works for Testament is likely that they have a decades-long working relationship.