Resolved by FrenchPress. 
(Original post below)
Sorry to bug you but I have some more questions;
I think you'd better make a new thread for this question. That would attract more people who might be able to help you for those issues. Perhaps @Aravisian can split this thread?
Thank you.
I have never used a Mac (Aside from a Macintosh back in the 1990's), so I really have no idea... But there are quite a few members of the forum that use Mac or have been using Mac recently. Hopefully they would have a better grasp of what makes Mac's startup Manager what it is and how to compare alternatives against it.
I did try a quick Net Search on this and turned up these. Perhaps they may help - you would understand if so better than I would:
https://itectec.com/ubuntu/ubuntu-dual-booting-os-x-or-macos-with-linux-without-refind/
Actually I was using macOS on my main desktop until recently. If you care to read it, I described the reason behind here: Praise for multitouch trackpad (Touche) in Zorin 16
My system is not made by Apple, but I assembled it to be 100% compatible with macOS.
Now to your questions:
You can use Startup Applications for that purpose.
If you uninstalled macOS, there is no way to access any application in macOS.
Thank you for the help!
Two more things;
You can set up an application to start up when Zorin Start up. I have Mega set to startup with Zorin OS.
Startup applications will automatically start applications when you login to Zorin. My Startup applications looks like this:
You can edit this list to control whatever apps you want to have opened at the login.
bootable device menu
Are you talking about Startup Disk?
There is no equivalent for that in Zorin. The only way to change the boot order is in BIOS/UEFI. But this is somewhat the limit of my knowledge - my system is "compatible" with Mac but it does not have a same BIOS/UEFI.
Thank you for explaining this to me.
I think I finally understood what you wanted to say 
You wanted to know whether you can use Startup Disk in Zorin to select the boot disk.
As I explained above, there is no such thing in Zorin (or any other Linux).
For that information, I think something like this would be helpful for you: (for dual-boot)
(for complete removal of macOS to install Linux only)
Thank you for explaining this to me.
Thank you!
If you have any other questions, please do not hesitate to ask. There might not be many Mac users in this forum but I will try my best to help you installing Zorin. As a former Mac user, I can attest that you will not miss out much switching to Zorin.
I think you misunderstood me. I already have Zorin installed. I am trying to figure out how to manage my bootable devices in Zorin. I have another copy of a different Linux distro that I want to test. I know I have already said this a decent amount of times but I am truly grateful for your help. 
Ah, OK.
Thanks for clarification 
If you are like me, who uses Apple trackpad a lot, Zorin is the best distro to have exactly the same multitouch function with it. I was a distro hopper and tried out all major Debian based distros before settled for Zorin.
I plan on permanently using Zorin. I am only using this other distro for my older laptop which cannot run Zorin Core.
I am trying to manage my bootable devices and I do not know how.
I think only people who can answer to this question is the Apple hardware users. My memory about a real Mac is rather rusty. I gave my real Mac to my mother-in-low (the only OS 88 years old can manage!) some years ago. Was it not pressing some key combination let you boot something other than the internal drive?
Thank you so much. This worked! This is such a big help, thank you! 
Congratulations to your decision 
If you happened to be using three-finger-drag in macOS, you might find this tutorial of mine useful:
