Who is a full stack designer?

It is important to clarify that the word full stack doesn't mean to do it all from designing, coding, database or management; it means that a person has developed multiple skills, which allow them to complete a design or a development project on their own.
A full stack designer is someone who can work through the entire life cycle/process of the design phase, starting from conception, research, and wireframes to UI design and Visual Design. They will be involved in the prototyping process using specific tools, such as Marvel, InVision, or Sympli, and will finalize the process by providing frontend style guides and UI Prototypes and animations using HTML/CSS, WebFlow, Framer.js, SVG Animation, and so on.

By involving the frontend code, I don't mean that you have to be a great coder like you are on design side, but you should understand how the frontend code works and what technologies are behind it; in the following diagram, I have added the frontend code on both areas, that is, Design and Development:

A good designer does not focus only on the UI or on the Design itself, but also on other things that are connected to it. A designer needs to realize the influence of different factors that can affect the product result. So, the more you are involved in these connected or related areas, the better a full stack designer you will become. And one of the biggest benefits is to involve developers from the beginning stage of UX.

The difference between a regular designer and a full stack designer is the ability to think of the bigger picture, because a full stack designer will be involved in the UX Process, design patterns, critical thinking, coding techniques, and a lot of other tools that will help them to finish the project, while a regular designer is involved in a specific area of design only.