I just went through this. As someone else mentioned, the first step was the prosthodontist made impressions of my mouth and was in close coordination with my surgeon about what the plan was so he could make the device as close fitting as possible. It was wired in place during surgery because it can then protect the skin graft and enable you to drink, eat and talk. Note: for me at least, this first one has no teeth. Shouldn’t be chewing on that side anyway as it’s healing.
At the post-surgery appointment (~10 days later) the surgeon unwired it so now it’s removable for cleaning. Trust me, it needs regular cleaning. The dentist will make a permanent one, with teeth, when treatment is finished and my mouth has healed. There was no point (for me) in making a permanent one until we know what all my mouth changes are as there is likely radiation and chemo in my future. Hope this helps.