Hi Jordan -- When you are downloading a map, the basemap used in the map is downloaded as the default basemap. However, you can copy a TPK to your device and then use it with the downloaded map. You might find this information useful: Can I use basemaps copied (sideloaded) to the device?
It is being changed to be a better experience and make use of content already on the device, but won't be identical to Classic as there are new offline workflows supported. This will be coming in a later release.
Okay...that is good to know.
As of now, offline areas have been nothing but an impossible hassle in v18.1.0. Offline use seems almost impossible at this point with on-demand downloads failing (and stopping) for no apparent reason, and for pre-made Offline Areas stalling out on download, or never completing the "generating" stage within Online. While offline functionality wasn't perfect in Classic, at least it was reliable. Hopefully we see this reliability replicated in the new version sooner rather than later.