Well...unfortunately you don't really own your work in most cases when working for someone else. Your best bet (as far as I know) is to describe the software and its impact on the business. A company will understand that you can't show them your previous code that is someone else's intellectual property: they wouldn't want you doing the same with their code.
Yea, this is definitely true. My comment about the code was more in terms of being able to actually run a live version of what you've done in the past, either as a demo or just a refresher for your own purposes.
And also, part of my concern that wasn't well pointed out, was in some cases those previous companies may not even exist. You couldn't even "point to the company" and say I worked there, on that.
For most business line apps (i.e., the majority of code that's being written in the world) there's small chance a developer would be able to run a live demo of their code.
If the code was freely available, it wouldn't meaningfully run without all the other connected systems and [semi-]live data - a live demo can exist (and often does, to be shown to potential customers or such), but it usually requires a bunch of dedicated servers and periodic work from operations team to configure & maintain it.
At least if the company doesn't exist they're not going to complain about it. Although whoever ended up with the IP in the liquidation sale might.
Running a live version of old software is often surprisingly hard, as it will depend on other old software or nonexistent systems. VMs make this a bit easier, but if it's a distributed or embedded system that doesn't work either.
