Your funnel process should be as simple as possible and removed from any distractions as possible (e.g. Header, Footer, etc). Your sole interest is getting the use through the funnel, so showing breadcrumbs + a potential stepper for multi-step funnel forms will be confusing. The more irrelevant components you show, the more crowded and page will be and the more chances for the user to get distracted and abandon the funnel.
In terms of dedicated multi-step form pages vs within the PDP page, it depends on your product, the purchase funnel, how involved the form is and the interfaces your users are accustomed with.
It's not uncommon to see booking processes for spaces or rooms to follow an e-commerce pattern of having the form within in PDP. Most likely it's for the user to reference some product details while booking.
It's also not uncommon to see more involved multistep processes having dedicated funnels. These include account creations, booking complex appointments and or forms used to personalized experiences.
The decision will depend on your product and the purchase funnel if your customer. Perhaps your form is simple and the user benefits from referring to the product details. Or perhaps your form is multi-step and requires more concentration.
Let me know if this helps.