What else would you call the well, stack, of technologies used to host a website? It's not a marketing term, just a way to describe the multiple layers. From hardware to network routing to DNS to tcp/up to http to https to HTML to JS to workers, and also Linux and PHP and a web server like Apache or nginx and a database like Maria or Postgres and maybe a cache like Redis or memcached and maybe squid and Varnish in front of it... that's a stack.
Jamstack is just a particular configuration where vendors take care of all of that and you can push up frontend code that auto deploys and that's that. I used to have to manage the whole stack and now can work purely in HTML CSS and JS and it is a DREAM. That's the magic of the Jamstack. Less devops and network infrastructure, more time to focus on UX and DX.
As for vendors, there's Vercel, Netlify, Gatsby, maybe a few others. Cloudflare, Deno, and Fly, Render and a few others have similar variations.
It's a really nice way to work...