The 20 years is the minimum time they'll do in prison.
After the 20 years they'll be eligible for parole. If, and only if, the board decides that it's safe for them to be released will they do so.
If they are released they will be in license. This means they are subject to prison recall without trial should they pose any risk to the public. The license may also impose additional restrictions such as regular therapy appointments, restrictions on leaving the country etc. These conditions will be imposed for life. Breaking any of these restrictions would also result in a recall to prison.
If the parole board decides that it's not safe to release them, then they'll remain in prison. They may very well remain in prison for their entire life.
This looks really cool! I have some time at the weekend so it looks like I'll be playing some Dominion of Darkness
Have you come across Pirate Software on twitch / YouTube? He livestreams his own game development, the community is very wholesome