What technology is used by grav

I'm planning for a CMS website, before I heard of grav my choice was WordPress but now I'm more interested in grav and I have few questions before starting.

1. What technology is used in grav.
2. How much I can scale up grav.
3. Is custom design and enhance funcanality of a dynamic website possible like creating themes from scratch.

Read the docs:

https://getgrav.org/features
https://getgrav.org/about
https://learn.getgrav.org/basics/what-is-grav

Plenty more if you google.