If you want to play different versions one after the other, that’s not too hard.
Open the songs one above the other and use the Time Shift Tool (two sideways black arrows) and shove track two so it starts after track one. then cut or edit each track so you don’t have to sit through three minutes of each song (unless that’s what you want). Repeat with three, four, etc.
You can use the MUTE and SOLO buttons to the left to listen to the tracks you want.
File > Export will mash all the active tracks together into one song when you’re done.
If you want to do overlapping and fading, then it’s harder.
I’ve tried something like this and even though the songs were in the same key, they weren’t the same length and I had a mashup that sounded like a series of bad mistakes.