Ukraine has offered those volunteers citizenship. Maybe they won't be able to return to the US but many won't want to. There is fellowship to be found in war. You fight and die with those around you, you live with that fellowship. People, like me, who don't go through that will NEVER understand.
When there is peace, as eventually there will be regardless of how long that takes, Ukraine or whatever country is there will have many veterans. They'll have to reckon with the horrors of war, what they did and what was done to them. But they will be able to do it together.
Some will go off and fight in other wars.
Most will try to rebuild their country and themselves with it. Whether they were born in Ukraine or chose to fight for her. Many will stay because all their friendships, built over years of war, are there. They will have the respect of the general populace (look at how the US treated WWI & WWII vets) and will be glorified for their sacrifices.
But it's not about glory, those who fight for glory give up, it's a common theme throughout history. Ukraine isn't fighting because they want to. It's a fight for survival.
At first I thought you were a troll for the sake of being contrarian. But it seems like you're coming from a place of genuine interest. So let me ask you a question: What would you fight for?
Family, friends, territory, honor, a lover? What would cause you to drop everything and go off to war? If the answer is nothing you might want to evaluate that. Maybe you're a Ghandi devoted to nonviolence but I'd argue even Ghandi fought a war. He marched across his country and was tortured for his efforts.
While you think I'm going to find a video someone made on YouTube, a kid who went to Ukraine to fight. Kid is a bit of a funny term, he must be approaching or in his 30s now if he's still alive. But he went off to Iraq with the US and was quickly disillusioned, then fought in Afghanistan as a volunteer for the Kurds. Now he's in Ukraine. He has an interesting tale and an interesting perspective and his own reason for fighting. I'd try to sum it up but I'll find the video and let him explain.