Is there a term to describe an apartment that opens directly to the outdoors vs to a shared hallway?
Garden apartment?
I’ve seen them called “motel-style” apartment buildings and I’ve seen advertisements where they are described as having “private entrances”
The terms I’ve seen are “exterior access” for opening to the outside and “interior access” for opening to a hallway.
Related question: are they regional choices, or just random chance?
When I lived in LA my apartment door went into a hallway. In Arizona, I’ve yet to see one that doesn’t go directly outside.
Colorado Springs has both kinds.
The subsidized housing I live has 6 apartments in a townhouse formation per building —12 buildings with a total of 72 apartments.
The buildings have four two bedroom apartments with with the kitchen and living room on ground floor and two bedrooms and bathroom on second floor. And then two one bedroom apartments one on top of the other at one end of building. All opening to outside of building.
I second this.
I suspect it’s going to be a matter of zoning and fire regulations, which will vary by jurisdiction. Exterior access may be safer by allowing immediate egress from the building (depending, of course, on where the fire is). On the other hand, I think they generally look cheaper, so tonier neighborhoods may not want them.
“Garden apartment” to me means an apartment at ground level that has a door to a backyard or garden, although that certainly might be regional. Would you call an apartment on the second or third floor a “garden apartment”?
And, I’ve also heard the term (or the similar “garden-level apartment”) to mean an apartment that is somewhat below ground level / street level – picture Laverne and Shirley’s apartment.
I’ve actually never heard of “a garden apartment”; just “garden apartments” to describe the style/a building with apartments that open to the outside vs a hallway.
An apartment that is somewhat below ground/street level is an English basement. 
(FWIW: I lived in various apartments in the Baltimore and DC areas from 1994 through 2003. Could definitely be a regional thing.)
I would. Garden apartments in Southern California are usually a 2 or 3 story U-shaped building surrounding a common area (sometimes a pool) visible not just from the ground floor, but also from the doors opening onto an open common balcony with access to the stairs.
Before I got married, I lived in a three-story apartment building, in suburban Chicago, that had open “hallways” (essentially long balconies), that each apartment’s front door opened onto. There was a stairwell at each end of the “hallway.”
I wasn’t aware of a particular name for that style, other than “it sucks during winter.”
Edit: a picture of those apartments, in Google Maps. It does kind of look like a mid-century motel. 
The low rent apartments as featured in Cobra Kai (the one instructor lived in one) open to a central courtyard in California. I’ve seen this in a lot of movies, I assume it’s a common California design.
Obviously security and weather are issues. In cold weather a sheltered hallway is a serious plus. For security, an open access is more visible, but more expensive since the hallway model allows apartments on both sides of the hallway for denser packing, whereas “garden apartments” (as good a term as any) don’t.
Not sure. Boston here. I live in a roughly fifty year old apartment building, now condo units, in a rental, and its basement units, use hallways entirely, while the first to third floors (it goes no higher) are hallway only, however, all the higher floor units have porches, such as my third floor one, and that gives some extra space and a nice view on either side, so one could call it semi-garden in that sense, I suppose, but the condo association is strict and punitive about things left out on porches, and they literally fine people, owners and renters, for leaving furniture on the porches, though they go easy on deck chairs and “cocktail” tables (no heavy stuff is I think what they mean). Also, they frown on using the porches to dry clothing, which irritates me, as I like air dried clothing, and clothes dried outdoors even in cool weather smell better, to me anyway, and have a special feel to them, while clothes dried inside, the bathroom , especially, never quite smell or feel right (but that’s me).
“Garden apartment” brings to my mind an apartment at or below grade, which may be a misunderstanding on my part. It doesn’t fit, in my mind, my current apartment which is on the second floor of the building and opens to an outside walkway. Wonder if the term definition varies by region?
In many ways, I’d prefer a hallway, especially in winter, to help moderate cold air coming in when I open the door, although hot air coming in when I open the door in summer can also be a problem. On the other hand, in more moderate weather the walkway immediately outside my door makes for a nice “porch” where I can sit outside for some fresh air.
If the OP is describing something like what @kenobi_65 pictured, here in Florida the open balcony / walkways are termed “catwalks”.
A building of that design, whether old or new, ratty or ritzy is termed a “catwalk building” or is “catwalk-style”. The opposite design with interior hallways like a high rise, is called an “interior hallway design”.
The tallest catwalk style building I’ve seen is about 14 stories, though more than 8 stories is darn rare.
If someone asked me to define a “garden apartment”, I’d say it must be single story, with its front door open to the outside world, and with no more than 2 or 3 apartments per free-standing building. Though that’s just the definition I’ve picked up by osmosis; that may be utterly bunk vs. what real estate pros think “garden apartment” means. Plus of course all the regional and international variations in any of this terminology.
That’s my definition, too. It may be smaller than the apartments above it due to space for laundry and utilites.
Maybe I’m not up on architectural terms, but to me a “catwalk” is one of those hanging walkways like you see in the movies where you are walking high above the backstage area of a big theatre.