What is most commonly used CAD software for aircraft design?
It really depends upon what you’re designing or analyzing, and what company you work for. In regards for structural design, at the major OEMs, CATIA V5 and V6 are the most common. This is particularly true for transport category aircraft. for example the Boeing 787 and 777X structures are all designed in CATIA. The defense and space business is a mix of CATIA and NX, with NX the preferred software for many space startups. Solidworks is commonly used by third party vendors and, strangely enough, popular with Scaled Composites. Cirrus designs all of its light normal category aircraft in PTC CREO.
What I would recommend is learn one particular type of parametric, feature-based, solid modeling software and you can use that as a springboard to other software types. If you can afford to take a course on a particular software app, feel free to do so, though they can become expensive. They are usually offered a standard curriculum at a major engineering department at a university.