How do the GDA2020 dynamic format(s) work (in practice)?
There is some detailed information here, on the ICSM page on datum modernisation.
Note importantly the two stage implementation:
The GDA2020 datum will result from a readjustment of the entire national geodetic network to a reference epoch of 1 January 2020. This will correct regional decimetre–level biases remaining in GDA94, and also introduce a systematic horizontal shift due to plate tectonics of approximately 1.8 metres compared to the GDA94 coordinates of the same point. A reference epoch of 1 January 2020 has been selected because it is anticipated that by 2020 the vast majority of positioning will occur directly in ITRF at the epoch of observation.
From 2020, it is proposed to transition from a conventional datum to a reference frame. The national reference frame, the ATRF, will accommodate coordinate locations that change over time and be highly accurate with respect to ITRF. Importantly, GDA2020 (or its successor) would also be retained in perpetuity, unless it became obvious that it was no longer needed. Appropriate national plate motion, deformation and distortion models will be used to propagate coordinates between any desired epochs. Given the tools and services accompanying the new reference frame, the geospatial community will be afforded the flexibility to adopt a fixed reference epoch (whether by national convention or arbitrarily chosen on a project by project basis) without compromising data quality and data integration that would otherwise be inevitable with GDA94 or GDA2020. The ATRF will be an authoritative realisation of coordinates and velocities of CORS and survey control marks throughout Australia. Once implemented, the ATRF will provide the Australian community with a sustainable, traceable, high–precision geodetic reference system capable of meeting the most demanding positioning requirements.
The TL;DR is that, at first, there will be a regular old 'at a time' coordinate system that shifts GDA coordinates into line with the current ITRF coordinates. And later, there will be a coordinate system that includes handling change over time.
I personally wouldn't worry about the dynamic component of Australian datum modernisation, as it's a long way from implementation.