Well, first off, for what you are asking you is 90% there when you work with ArcGIS Online with a free developer account and a $100 personal ArcGIS Desktop license. So the very first thing to do is get really really good at setting up and using ArcGIS Online. (I have not done this because I use Portal and Enterprise 99% of the time!! But seriously they are pretty close.)
The details where they differ won't really come up in an interview... unless I suppose that's the question. "Tell us how they differ!" Interviews are nightmarish to me. Broad stroke... one runs here and the other runs there. Where was I?
If you can use (for instance) Experience Builder and make a few nice apps that I could look and show me how you manage the data for them you'd be ready for a job here regardless of whether you had learned on ArcGIS Online or in Enterprise.
I personally enjoy programming and system level work so what I did was pay the big bucks for a developer license for a year.
I had to search for it, the link is https://www.esri.com/en-us/arcgis/products/arcgis-developer/buy
I was going to say that $1800 was an absurd amount to pay but then realized other people here are suggesting $1500 for a 2 day class of being rushed through a chute is worth it. I don't learn much in those classes. I have taken 5 or 6 now. Usually by the time I get a chance to apply the knowledge, it's mostly gone and I have to learn it again on my own. I need continuous access to the tools to learn.
I don't know what motivates Esri, I think they want people to learn to be web developers or something so they are pushing in that direction. I could not find any references to the developer license on the developer web site.
It really depends on where you already are on the curve. In real life you probably won't be the one installing and upgrading server stuff but if you want to learn that then you will never ever get it in a 2 day class. You have to do it like 20 times. You have to drop it when you get confused and come back to it when your brain has had recovery time for a few days. So maybe $1800 for 12 full months of unlimited access is actually a good deal.
Then of course on top of that you have to have a computer to run the server on, and that sets you back another $1000 or so. You can also rent space for example on Amazon AWS. Personally I hate "don't-worry-we'll-bill-you-later" because I don't like to be surprised by a bill at the end of the month.
The $1350 developer plan does not include much beyond what you get for having a free developer account. The $1800 plan includes ArcGIS Enterprise Advanced which is what I wanted. It includes ArcGIS Pro BASIC which when you are trying to learn Enterprise is quite useless so you still need the $100 personal license.
If you are not trying to learn the nuts and bolts of how the components work then you could install once and get on with learning Portal and Enterprise as a user. Then you'd learn as I have that it's pretty close to ArcGIS Online.
Note that any of these contracts also gets you technical support, so if you are experimenting and really get stuck, you can still ask questions. If you are a developer and you PAID FOR IT you get to open tickets. I'd also love the hear that story from a potential candidate - "I know how Esri tech support works because..." I've been learning how to use support for a couple years now. Sometimes it helps. Sometimes they hang up on me. 🙂 Well, just once. She was stumped!
I'm currently trying to figure out what happens next in my career. Do I "retire" in a couple years from my county job and continue using Esri tools or do I move in a totally new direction? When someone else pays for Esri it's not as hard a decision.
Good luck
