I was given a template to work with that I can modify but I lack the necessary understanding about the pipeline.
If anyone has the time, I would appreciate (also partial) explanations how the pipeline constructs the public folder and how I could change the project so I can put other files in the public folder, too.
I want to put a js file into the public folder in case that is relevant.
The project is currently structured like this:

├── README.md                       -> The file you are currently reading
├── index.html                      -> Single HTML file for the wiki
├── package.json                    -> Manages project metadata and dependencies
├── src/
│   ├── components/                 -> Components (Like Navbar, Timelines, ...)
│   ├── App/                        -> Main React application container
|   |     └── App.tsx
│   ├── contents/                   -> Page components for the website
│   │   └── *.tsx                   
│   ├── main.tsx                    -> Entry point of the wiki application
│   ├── pages.ts                    -> Page definition and path mapping
│   ├── utils/                      -> Utility functions
│   └── vite-env.d.ts               -> TypeScript definitions for Vite
├── tsconfig.json                   -> Configures TypeScript options
├── tsconfig.node.json              -> TypeScript settings for Node.js
├── vite.config.ts                  -> Configuration for the Vite tool
└── yarn.lock                       -> Yarn lock file for dependency management

This is the .gitlab-ci.yml file:

image: node:18.20.0

build:
  stage: build
  cache:
    - key:
        files:
          - yarn.lock
      paths:
        - .yarn-cache/
  before_script:
    - echo 'yarn-offline-mirror ".yarn-cache/"' >> .yarnrc
    - echo 'yarn-offline-mirror-pruning true' >> .yarnrc
    - yarn install --frozen-lockfile
  script:
    - yarn build
  rules:
    - if: $CI_COMMIT_BRANCH != $CI_DEFAULT_BRANCH

pages:
  stage: build
  cache:
    - key:
        files:
          - yarn.lock
      paths:
        - .yarn-cache/
  before_script:
    - echo 'yarn-offline-mirror ".yarn-cache/"' >> .yarnrc
    - echo 'yarn-offline-mirror-pruning true' >> .yarnrc
    - yarn install --frozen-lockfile
  script:
    - yarn build
    - cp -a dist/. public/
    - echo '/* /index.html 200' > public/_redirects
  artifacts:
    paths:
      - public
  rules:
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

I read a few articles and basic instructions about pipelines, gitlab CI, vite and yarn. Unfortunately for me, this is not a very basic pipeline and it seems like it is always assumed that it is clear what expressions such as p -a in the yml file mean.

Using the classic “delete something and see what errors arise to figure out its purpose” approach, I figured out that - echo '/* /index.html 200' > public/_redirects makes all links to files in the public folder go to the index.html page (I think…).

And that cp -a dist/. public/ is necessary to find all the files to build the website with.
Therefore, I tried putting the js file into the project structure as follows

├── README.md                       -> The file you are currently reading
├── mapscript.js
├── index.html                      -> Single HTML file for the wiki

Alas, that did not lead to it being found.