The Material Design 3 guidelines expresses how content should flow with foldable with slight differences compared to a tablet, for example:

https://m3.material.io/foundations/layout/canonical-layouts/list-detail

But you can mostly get away with a standard, two-pane layout as you would a tablet, though the center padding may be off.

You probably want the center 24dp spacer to be where the crease is, but it gets a bit complicated if you want to use a Nav Rail since it'll offset your layout by 80dp. Here is a web based demo of what the configurations can look like.

Yes and no. For some apps, the developers have it set to scale to larger screens or have a certain layout when it detects that the DPI surpasses a certain threshold. This is really how they just get all of these apps to scale for traditional smartphones in general since there are many different screens sizes out there. What I'm referencing is that Sync is one of the few apps that goes from a traditional phone app when folded shut, to a dedicated tablet layout when the phone opens. Other apps such as Outlook, Gmail, Relay for Reddit, YouTube, Google Calendar, and some more apps do this as well.

Right now, Samsung's Foldables have the ability to force apps to expand to the aspect ratio of the inner display for any application. The Pixel Fold, however, doesn't include this feature so there are some apps that genuinely just stay in a traditional aspect ratio of a smartphone and doesn't expand to take advantage of the screen real estate on the bigger display (if the developers didn't build it in). The good thing about this is that for the Pixel Fold, because of the aspect ratio of the inside screen, you can have two regular sized apps splitting the screen side by side and it looks really good.

Google is doing this though to force app developers to start making tablet optimized apps and is even incentivizing developers by pushing their apps to the featured page of the Google Play Store once their app fully supports tablet displays. Apps that don't make the change however, will have a message displayed for us foldable or Android tablet users that will say something like, "This app may not be fully compatible with your display," when we go to install it.

A lot of information but yeah that's the current foldable situation. I've been using foldables since the first Galaxy Z Fold and am now on the Pixel Fold. Absolutely love folding devices and will never return to a traditional slab phone.