Add your clips all at once or one by one. They line up in the order you picked them; use the arrows to move them around. That's the order they'll play in.
The frame size comes from the first clip. Every other clip is fitted inside it, with bars if the shape differs, so a vertical phone video is never stretched or cropped. Clips without sound get silence so the audio of the others stays in sync.
Tick “Smooth fade through black” for a short dip to black at each join. It makes cuts between very different scenes feel deliberate.