Every airline has specific limitations, memory items, etc.. that may be different even for the same type. Everyone reading this who has actually worked for an airline knows this. They become limiting and actual law just by virtue of being in the airline's SOP / GOM and being approved by the FAA. In the 121 world, manuals pretty much trump everything you could get from online or a pilot shop. There is no harm in getting to know an airplane in general before you work somewhere. But don't really start studying i.e. memorizing stuff before the airline / operator you are going to work for actually sends you what to study before your class date.