Have you never used the Arch User Repository?
The lazy persons way:
yay -S thorium-browser-bin
The right way:
- Goto AUR (en) - thorium-browser-bin
- Download a Snapshot (upper RH of page)
- Goto the directory you downloaded it to.
- Extract the PKGBUILD (right-click, extract)
- Open a terminal in the PKGBUILD’s directory.
- Type
makepkg -siin the terminal and hit enter. - Tada!
EDIT: For users new to Arch & Arch-based Linux:
The second way is the correct way to install AUR packages. Nobody needs a “helper” like yay or paru or whatever to install an AUR package. Learning to do things the proper way in Arch goes a long way towards establishing a lasting relationship with Arch. Do I ever use a “helper”? Oh, hell yes, I’m as lazy as the next person. But here’s a hint; the next time an AUR package fails to build using a “helper” then try the correct way. You might be surprised. 
Authority: Arch User Repository - ArchWiki