Apple Business Manager (ABM) establishes chain of custody for hard (Macs, iPads, iPhones) and soft (apps available via the App Store) assets. ABM is not an MDM and does not manage devices. It enforces trust of ownership of hardware and software as well as establishing a trust to your identity provider (managed Apple IDs). Apple has an MDM called Apple Business Essentials (ABE), but I am not a fan as it is too limited when considering Mac device management.
On the surface, the answer you are liking looking for is that you need to (1) sign up for ABM before you buy the hardware, (2) pick an MDM, and (3) setup the MDM to enroll, configure, and manage your devices. I feel that all Apple customers should be signing up for ABM regardless of MDM intent. There are many MDMs so you should research them to pick one that fits your needs. I am a fan of Jamf but I am just a random person in the forum expressing an opinion. There are many others including ABE from Apple, Mosyle, Intune from Microsoft, Hexnode, Airwatch, Kanji, the one from Meraki, and many others. Pricing and capabilities vary from MDM provider so understand your needs so you pick properly. No one wants to change MDMs.
Now, regarding the order. Ideally, you will sign up for ABM first. Then you will tell your reseller (or Apple if buying direct) that you have the ABM tenant. This will allow you to exchange Org ID and reseller ID (or Apple customer number) so that all the hardware purchases are automatically associated to your ABM at time of purchase. If you buy the equipment first, then you will need to use Apple Configurator to side load all the units - that sucks when you are talking over 100 units.
Then, you can volume purchase/license the required Apps in ABM. Export your DEP (device enrollment program) and VPP (volume purchase program) tokens from ABM and import them into your MDM. Create an enrollment policy and then start firing up your devices. They will automatically enroll, automatically deliver apps without logging in with an Apple ID, and enforce all the setting you need for a safe and targeted deployment.
Now, the one problem you may have... Crestron Go is listed as supporting In App purchases. If you need to add functionality to the app beyond the basic, then you will not be able to use ABM to license the software. ABM has no mechanism for handling In App purchases and an In App purchase (which requires and Apple ID) will only work if the app is installed using the same Apple ID. If you have no need for the In App purchases, then you will be able to distribute the app via VPP with no user interaction. The MDM provides this function after ABM handles the license link in the chain of custody.
So, in summary, (1) ABM is required to do volume licensing and an MDM is required to assign the app to the device. (2) Any MDM that supports ABM will work to deliver the app to the devices. Remember, in the US there is one App Store so the single source of apps makes delivery easy and safe. (3) Basically, the same answer as 1. ABM is used to acquire the license to the software. The MDM assigned it to the device and aids in keeping it up to date.
Assuming you will never need the In App purchase function, you will want to sign up for Apple Business Manager, inform your reseller once ABM setup is complete, place your order ensuring that you are buying from a DEP capable reseller so the devices are associated to your ABM at time of purchase, select and setup an MDM considering your required apps, your profiles, and your management approach, enroll the devices through automated enrollment, manage the devices using the MDM.
You can consider reaching out to the Apple Consultants Network if you need help. Many MDM providers offer professional services to help new to MDM customers get started. For your first time, it may make sense to get a specialist to get your deployment done right and quick.
Hope this helps.
