Trying to get more understanding of Docker

I know this is possibly a really silly question, so I will ask it anyway.

Wappler gives easy access to things like portainer, and traefik which i could install manually if I were not using Wappler, from the library of containers on https://hub.docker.com/search?q=&type=image
I also see in that library, mysql, mariadb, and redis among many others.

I assume Wappler just took these and made them easy for all of us to use without us having to really know how.

Using portainer after a docker system prune it reports
2 Stacks, 4 Containers, 6 Images, 4 Volumes, 4 Networks

The 4 containers make sense to me

1. Web (NodeJS Application container)
2. DB (MariaDB / MySQL Application Container)
3. Traefik
4. Portainer

The 4 Volumes make sense too, as most of the containers need a volume to store persistent data

1. db-volume
2. user_uploads
3. letsencrypt (My SSL Certificate)
4. portainer_data

The 6 Images also seem to be relevant to the containers and volumes above by nature

1. mariadb-10.5
2. portainer
3. traefik-v2.2
4. node-14
5. db.latest
6. web.latest (Judging by date created, this seems to be the created image with each deploy)

Questions:

1. I would like to know what the 2 Stacks are. In fact what are Stacks all together?
2. Why 4 networks, and not just one?

Larger Question
Can I install more images i may want to use in my project?
In https://hub.docker.com/search?q=&type=image&page=2 I can see things like sentry, phpmyadmin and even wordpress which I may need from time to time if my client wants to add a blogging section to a site i have already created with wappler and docker.

If I can install these images, what is the best way, with NPM, or the new homebrew integration, or could I use portainer, which seems to have a GUI in the images section?