TL;DR: I think they’re going for UUIDs as concept identifiers and their file format will be inspired by the IMDF (vocabulary very, very similar to OSM Wiki, but uses “.” over “:” / “_” on keys)
Based on quick casual browsing on Twitter for “overture maps”, then people proud about this, I found someone’s from one of these company days before mentioning this (which might give a hint on their mindset to push for broader adoption of whatever data exchange format to sideline OSMF over time):
From their “solutions” one is the GTFS from Google (pointer as success case for bus routes) and a new concentioned data package, Indoor Mapping Data Format, which seems to be around since at least 2019. But is from Apple, not AWS/Meta/Microsoft/TomTom:
At the https://www.ogc.org/pressroom/pressreleases/4415 explains a bit how this has been standardized, but I’m somewhat a bit disappointed if Overture Maps Foundation will either use IMDF done by another company or create a conventional in similar way. From the “war” on open vs closed specifications, I remember the case of Office Open XML had a major discussion, even an ISO, to brag about be an open standard with wider discussion, but OMF (if going this path) will effectively have an “open standard” not more than an informational IETF RFC. And from this link, a paragraph:
An OGC Community Standard is an official standard of OGC that was already available as a widely used, mature specification, but was developed outside of OGC’s standards development and approval process. The originator of the standard brings to OGC a “snapshot” of their work that is then endorsed by OGC membership so that it can become part of the OGC Standards Baseline.
So, this approach on file format is actually less than the OpenStreetMap Wiki: the actual “points of interest” are poorly defined, if defined at all over mere key use in en-US. Pretty lazy job, I mean is not even monolingual. The JSON/Geojson is not even JSON-LD/GeoJSON-LD. Not just this is less semantic, but seems that the whole “Global Entity Reference System” is a buzzword for long random UUIDs that any tool can generate but are unlikely to conflict with each other (actually, "globally unique identifier GUID are another name used for UUIDs).
Despite them saying in the FAQ they aren’t focused on community, and informally we know here they’re sometimes upset with DWG not allowing mass changes, from the “OGC Community Standard” thing, plus all the idea of trying to appear friendly to OSM, seems otherwise. Also even GTFS from Google had very slow adoption despite having no competition of file formats, so Overture from medium to long term attempt requires developers to promote use of their exchange format (if they don’t, it will make them work very, very, very manual, unable to scale up).
Anyway, they’ll be very susceptible to public criticism about technical implementation (because otherwise their data would be more outdated pretty quickly). But if they’re going for an IMDF-like approach, from their way to think like layers, to even the way they would do conflagration at later stages, becomes predictable, regardless of they avoiding leaking any code before going public.