If you don't plan to use an AUR helper, you can simply build the package from source by running(using the package you gave here as an example):
git clone https://aur.archlinux.org/packages/intellij-idea-ultimate-edition
cd intellj-idea-ultimate-edition
makepkg
pacman -U intellij-idea-ultimate-edition-2023.2.1-x86_84.pkg.tar.zst
If you do want an AUR helper, I'd highly recommend aurutils, as it integrates directly with pacman, instead of trying to just act like pacman.
After you've installed it, Aurutils requires you to setup a local repo, and they have instructions for this and more advanced usuage in the bundled man pages
but the basics of it are:
Add this to the bottom of /etc/pacman.conf
(the local repository root is /home/custompkgs here but you can keep it in any dir you want to)
[custom]
SigLevel = Optional TrustAll
Server = file:///home/custompkgs
Create the repo root and database:
sudo install -d /home/custompkgs -o $USER
repo -add /home/custompkgs/custom.db.tar.gz
If there are any built packages, add them to the database:
cd /home/custompkgs
repo -add -n custom.db.tar.gz *.pkg.tar*
And then sync pacman by running sudo pacman -Syu
To install packages with aurutils, just run aur sync {name of package}
and then pacman -S {name of package}