Chris, any 'hard' areas are covered in the card (sourced from the boxes the kits come in), which is scored 'in situ', leaving the 'grass' or 'bare earth' areas clear. This gives the impression of depth between the 'concrete' and grass, with the latter added from the mats, static grass or whatever, once the 'concrete' has been painted and weathered. Where variations in surface are required, such as hillocks, slopes etc, these are built-up using either layers of card to form contours, or the same using expanded polystyrene packing material, as described in Part One of the Diorama Guide I posted some time ago (I still haven't got around to doing Part Two!!)
Note that I don't normally have 'grass' or 'weeds' growing out of the expansion joints - this might happen when an airfield is 70 years old, and abandoned, but, during the period depicted, these joints would normally be filled with tar. However, at the edges of, for example, a dispersal pan, I may add the odd bit of weed or grass, particularly if there are 'cracks' in these areas.
The nearest 'railway scale' for 1/72nd scale is '00', but I use various scales, as, for example, a large '00' barrel might work well as a smaller '1/48th scale barrel, and so on. It's a case of what can be adapted to suit a particular need, and useful items might be found in various scales, in ship modelling, car and military vehicle accessories , doll's house accessories and some toys, as examples, others being scratch-built as needed.
The majority of my models are 1/48th scale, although in the posted pics, the Bf109G is 1/32nd scale, and the Lancaster, Halifax and Dakota are 1/72nd scale, built for commissions. Figures are from various sources, with, for example, the crew and ground crew in the Boston diorama, being modified Tamiya 'Panzer Servicing Crew', with a couple being modified Japanese aircrew figures, from a Tamiya kit, donated by Karl (rochie).
The 1/32nd scale figures are modified from Airfix 1/32nd scale 'Multi-Pose' German Infantry and a (very)old Airfix slot car figure.
Note that 1/35th figures, accessories and vehicles do not look right next to a 1/32nd scale aircraft - the difference in scale does show!!