Arch Wiki is your friend and a fantastic resource.
Learn pacman inside out, best package manager in penguin land.
Understanding how to define (PKGBUILD) and build (makgepkg) packages definitely helps.
Arch User Repository.
Fantastic tool, so many packages in one location, easy to use.
Just be careful though as all these packages are user maintained. Each AUR package has its own web page. Read it first. AUR package comments, PKGBUILDs and source help determine a level of trust in an AUR package.
As Arch is a rolling system you’ll need to maintain it to keep it healthy in the long term. Install once, maintain indefinitely.
If you are looking for apps to do certain tasks, this may help.
If you need / want access to older lts kernels and lts nvidia drivers use this.
Unofficial repo for older LTS kernels
And finally you’ll need a tested backup / restore process; and be familiar with chrooting into a system through a live environment to fix issues that require it.
You won’t need these often, but Arch is bleeding edge so you’ll probably need them eventually.