A somewhat simplified answer.

When both elevons goes up they will push the back of the plane down. This makes the nose start to point upwards, or as we say the plane pitches up. This increses lift and so the plane goes up. So the sequency really is: elevons up --> back en pushed down = pitch up --> increased lift --> increasing height.

If one elevon goes up, and one down: the one going up will create a pressure downwards, the one going down will create a pressure upwards --> roll towards the eleven going up.

A somewhat simplified answer.

When both elevons go up, they will push the back of the plane down. This makes the nose start to point upwards, or, as we say, the plane pitches up. This increases lift, and so the plane goes up. So the sequence really is: elevons up --> back end pushed down = pitch up --> increased lift --> increasing height.

If one elevon goes up, and one down: the one going up will create a pressure downwards, the one going down will create a pressure upwards --> roll towards the elevon going up.