How do you secure Arch?

But that's the nature of the beast. Unless one defines their threat model^[1], there's an ever-expanding list of improvements one might apply to enhance security; with -at some point- (mostly) diminishing returns and we've yet to talk about the amount of comfort that's sacrificed along the way. Therefore, before you do anything else, define your threat model. Afterwards, try to apply step-by-step whatever is required to protect your assets to a degree you're comfortable with^[2]. If, however, this seems like too much work for you, then consider either one of the following:

• Just go on with your life as if you hadn't become security-conscious. If you're just a random person that doesn't store anything valuable on their device in the first place and isn't a possible target to more sophisticated groups for whatever reason, then even in the worst-case scenario you can just reinstall your system and be done with it (assuming your home network hasn't been affected by malicious actors).
• Reconsider how you want to consume Arch and if Arch Linux is even the right distro for you. Distros like Fedora and openSUSE are better known for maintaining good security defaults and try to ever improve themselves in this regard. Sure, sometimes some of these changes are applied to Arch as well. However, by its very nature, Arch Linux is more akin to a blank slate.Thus, if you actually know what you're doing, then it's easier to get Arch Linux to wherever you want^[3]. But, becoming that knowledgeable is easier said than done.
• If you really like Arch, but also really care about your security, then it's probably best to look into the most impactful changes (security-wise) with the least amount of work associated to it. Simply not using packages from the AUR is one such change for example, if you can afford it...

1. Digital security and/or cybersecurity is actually just one part of it.
2. In terms of initial setup, (possible) maintenance and lost comfort.
3. This even applies to hardening your system.

This page is really to help you defining what would be of concern for you. There are too many use cases and security measures will differ greatly. It is not a step by step guide.
At the very minimum, since your firewall is already setup, just make sure to keep your firmware up to date with fwupd if your machine supports it and follow the basic good practice below:

• regularly update your packages
• do not install intrusted packages
• use strong and unique passwords
• run your app tests (if any) within a sandbox

If you need AppArmor as you mentioned. You should really invest efforts into it. ArchLinux is by nature a demanding distro for its setup. That being said once installed and activated (i.e. litterally 2 commands to run) you should be good to go unless you want to setup additional profiles.

Once you figured out how to meet your own security needs, you can start the same lengthy process to address your privacy needs ;)

Yes. AUR package maintainer(s) are additional people who can add malicious code (or someone else can by compromising their account).

I know that almost nobody treats it this way but the number one rule of AUR is that it's pretty much all untrusted, by definition.

I prefer Flatpaks and for them I just settle for the build in controls. Hope they get better, but restricted FS access is mostly enough for me tbh.