The Schematic Editor
Most electronic desings start with the schematic editor, placing some parts and changing their values. Not every designer is thinking about concrete parts (Manufacturer Product Number – MPN), their physical case dimensions, and the availability when designing the schematic.
What you really have to consider is, what component technologies you might want to use, in order to meet your assembly requirements. Threrefore you might have to look for TSSOP Cases instead of no-leaded QFN packages. Most active parts you will get in different cases directly from the distributor, only take a look into the Datasheet, all possible packages of one part are clearly shown, often with dimensions and order codes (Maufacturer Part Number). As an Exaple you might take a look into that Datasheet of a DAC. You could freely choose between TSSOP or QFN Package, the costs and the availability at the distributor is nearly the same.
When using parts in the schematic it makes clearly sense to think about what concrete part, also what passive components like resistors and capacitors, you will place in your layout, especially if some cases may have long lead times.
So when you already know what parts you want to use its time to set up your CAD Library properly.