In transposition of great arteries (TGA), the pulmonary artery arises from the left ventricle and aorta arises from the right ventricle.
Aorta is anterior and to the right of pulmonary artery in D-TGA, which is the variety which we mean when we say just transposition of great arteries. L-TGA is a physiologically corrected transposition with aorta to the left of the pulmonary artery, also called C-TGA.
In L-TGA, in addition to ventricular arterial discordance, there is also atrioventricular discordance so that systemic venous blood flows from the right atrium to the morphological left ventricle and to the pulmonary artery. Similarly, pulmonary venous flow reaches the morphological right ventricle from the left atrium and further into the aorta.