I bit the bullet and ordered a Mac Mini. This will be the desktop I do all my work on and the Air will simply charge in the office be used when I need portability. (with me so far? 
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So, for that to work as I’d like it to, everything needs to be in cloud. Is it simply a matter of making sure everything I create and save is to the cloud. Is there any tips, tricks, hidden traps I need to be aware of?
Sorry if this is the simplest question of all time! 
I have a similar setup, a Mac Mini with 16GB connected to a 34" monitor. I do the bulk of the work on this as I like the bigger monitor. I have an M1 MBP that I use when I am not in front of the Mac Mini. I am retired now so I do not have to go to work but I still use MBP 40% roughly for writing blog, checking emails, etc. I am writing this on this MBP
I have a family shared 2T iCloud subscription, so I try to use apps that sync between the two Macs, iPad and iPhone. I prefer apps that support cross device sync. I also use Google Drive for Gsuite files that I collaborate with my family and friends. In terms of cloud services, I prefer Dropbox, Google Drive and iCloud (in this order) but the biggest cloud subscription is iCloud mainly because of device backups, apple photos for my family members, otherwise I would downgrade to the next lower tier
I rarely use hardware (USB, SSD, etc) to share files between devices.
Not sure these are the details you are looking for. Happy to provide more if necessary.
Some of the cloud services, including Apple, are trying to ‘help’ by making the default configuration to keep many/most files “offline” and in the cloud to save disk space.
Apple does that with your iCloud Drive with “Optimise Disk Storage”.
That’s fine as long as when you are off the internet and you need a particular file that is not available, or if your app expects all files to be local and they aren’t local.
Read the documentation of the apps you use, and how your cloud service provider works.
I also use iCloud to store my files. My Documents, Desktop is automatically on iCloud. I even use it to share files. I like that I can set an expiry for shared files on my iCloud or if they can also share it to other people.
Be aware, that the cloud is not a backup, and if you are using a setting, to optimize the disk storage (or a name like this) you could end with (at least some of) your data only to be in the cloud, and without a “real” copy on your Macs.
You should be aware, that this situation means also, that TimeMachine, or an other Back-Up-Service, will be unable to backup those data, as they are not on your system.
So I always have one Mac, that maintain all iCloudData as a hardcopy, without optimisations for the storage, to have one, I could backup.
I could be wrong, but I thought iCloud stores the backup of iPhones, etc?
At least, this is what I read from Apple
No, you are not wrong on that, but in this particular case, iCloud is acting as a backup.
The “normal” use case of iCloud, storing and syncing your personal folder, on the other hand is not a back-up, like on the most other cloud storage solutions.
If you, for example, accidentally delete a file, it will also be deleted on the iCloud (yes, you can often get it back for 30 days, but not for “30days and 1 second”). While you still can get it back from a TimeMachine (if your storage last long enough!) and the file was on your Mac a regular file, and not only a download link pointing to the iCloud.
Yes, there is iCloud Backup for iPhones and iPads. But not for Mac (yet?).
The “cloud is not backup” is a saying that I use to try to explain that just because files are in the cloud, that doesn’t mean they are backed up. The “cloud” is just someone else’s computer. For example, a backup is checked for data integrity, is designed to be easily restored, has versioning, etc. I’m speaking in generalities, of course.
So use iCloud to keep files in sync between your Macs (see Desktop and Documents in iCloud), but ALSO have backups of both of those Macs. Or, at least, the one that has ALL of your data on it (the desktop, I presume).
I’d use Time Machine on the desktop Mac, and Backblaze on the laptop. And possibly Backblaze on the desktop, too, if I don’t have a way to easily keep an off-site copy of data.
Thanks everyone. A few things in there I need to take note of. Especially making sure I have access to the Cloud or everything I need is on the Air before I head out. Plus the backup side of things. It was naive thinking that if everything was on the Cloud it was “backed up”.
Hey @margaretamartin, @Ulli I’ll need to look into backblaze, hear about it all the time but never taken the time to investigate. Thank you!
If the Air doesn’t have enough SSD to hold all your documents in the cloud you will need decent internet connection where you’re working to be able to download the files. My older Air has a user upgradable SSD so I put in a 1tb drive to make sure I always have my files with me.
I’ve been very happy with BackBlaze for an offsite backup. All my test restores have worked with no issues.
It’s really great to have a wire-free backup system for a laptop. We all have good intentions, but it’s hard to remember to plug in an external drive frequently for backups when the beauty of a laptop is freedom of movement. As of now, Backblaze is the only game in town (if you want something simple and reliable), and with it you get not only in-the-background backups any time you have internet connection but also an off-site backup.
If the Air will not have a copy of all of your documents, you definitely want to read through Backblaze’s description of what, exactly, will get backed up. It can get confusing because services like iCloud make it appear that all of your files are on your internal laptop, but if you’ve chosen to “optimize”, some files may be removed to make room. And obviously, Backblaze cannot backup what isn’t there!