A mid or second layer is normally a lycra jersey of some description.
And in fact, if it is not really hot or freezing, these would be your only layer. Again here, many options both low and high-tech exist. And more recently, due to better technology, generally the middle layer tends to be a less bulky item than in previous years. If your base layer or layers have been well-chosen, you would not need a very heavy weight second layer.
Should be worn tight and often cut for your position on the bike. There are many options regarding thickness and material type, at the cutting-edge options can even be more or less aerodynamic. For all second layer options pockets on the back are very useful, and I prefer a full-length zip.
That layer exists in myriad options to suit your preferences and budgets: long sleeve, short sleeve, half zip, full zip, pockets, pockets with zips, different high-tech options with panels of different materials of different qualities. Windproof, thermal. And in recent years many companies have been working on a less race orientated designs like Gobik and Santini.