Military engineers focus on battlefield-relevant construction and demolition techniques and aren’t routinely deployed on civilian engineering tasks during their readiness cycle. One exception would be military assistance in emergency situations, such as quickly erecting a bridge to replace a flood-damaged one. For the average soldier, their trade-specific training would be measured in months, so the only opportunities to pick up deep domain knowledge are limited. Career engineers might get deeper knowledge but I suspect they would still focus on battlefield-specific competencies.
In the USA the US Army Corp of Engineers is heavily involved in public/civilian infrastructure. For example, see this article on their involvement and attendant controversy in Hurricane Katrina (2000s New Orleans): https://en.m.wikipedia.org/wiki/U.S._Army_Corps_of_Engineers...
I won't comment on training because I suspect that varies too much internationally to be worthwhile - but of course they focus on battlefield-relevant tasks; are you saying that wasn't also true of the Romans? I don't know, I just assumed (and asked if) they were pretty similar.
The difference is that Roman soldiers could build roads and fortified camps as part of their day job using skills similar to civilian Roman construction. Today, however, building a modern motorway, or a high rise building, would be way different to battlefield engineering.
