I bought the dead-tree edition of this book, which may answer your initial questions...and leave you with even more:
https://www.amazon.com/gp/product/B08NVHXLZM/
It is a summary of various Facebook barndominium forum posts (I don't do Facebook!), with links to additional information on the web for financing and stuff. Good overview, explaining how construction loans work, etc. They make an argument for including the land purchase into the construction loan, to make the bank feel better about writing the loan. Not full of details, but rather an intro to the process.
Since we are using a HELOC on our nearly-paid-off current house to self-fund the retirement project, I'm not overly concerned with construction loans and making banks happy.
Living in a town in upstate NY that identifies itself as agricultural/rural, I just spent the last year on a town committee writing a new 10-year comprehensive plan. The first thing that I checked out in SC was their county comprehensive plans, to see what they identified as their problems in their
SWOT analysis and if a barndominium would be excluded off the bat with no discussion. The county plan where we just bought (SC midlands) was just updated, and their biggest gripe is dilapidated mobile homes. Other problems are dilapidated stick-built houses in the town centers, and lack of reasonably-priced homes for lower income families. Maybe I can go into business selling barndominiums there?
On the topic, a mobile home was made before the mid-1970's, when HUD came out with construction/materials regulations. Manufactured homes are the same type of thing, but made after the HUD regulations were adopted. A modular home is built off-site in pieces, delivered by truck, with final assembly at the home site. These distinctions are important, because a steel barndominium should be considered more like it is between modular and stick-built...and not a manufactured home. When you start reading zoning codes, these are each specific building types and not interchangeable terms.
Since we are not in a hurry to build, I was thinking about putting up a small temporary cabin-type kit so we could have a place to stay for a getaway weekend and during the construction process. Hence, why I went digging into the zoning for various temporary structures in the previous paragraph.
The strange thing for me coming from the northeast is that a lot of counties down there seem to have just chosen the international IBC or ICC building codes rather than create their own. As far as I can tell, all you need to do is buy a steel kit that's certified to those standards and you're apparently good to go. Guess I'll be finding out in a few years when we reach that step!
My DW told me that she would divorce me if I didn't build her a 10-car garage within 10 years of our wedding day. We are already past the deadline, but she came up with the barndominium idea and insists on a 5-ton lift and slop sink in our new living room. Since we already live in a small house surrounded by 7,000+ acres of state forest, the whole thing isn't a huge jump for us.
I have no fears of serving as my own general contractor (GC) for the project, but will have to face reality and hire someone down there to act as a GC. The distance is way too far to properly oversee a construction project, and my Rolodex is empty for locating reliable contractors down south.....