Tools and Common Workflow

Eeschema includes a "library manager" tool to manage components.
If also offers a graphical editor to make your own components
Additionally, each component must be associated with a footprint
This is done while placing components
The "cvpcb" tool is an alternative, separate tool, now discouraged
Pcbnew, similarly, has a "footprint manager". And and editor.

So the standard workflow looks like this:
eeschema: you draw the schematic diagram, assign footprints and save
pcbnew: it reads the netlist and footprints, you route the PCB and plot gerber files
gerbv[iew]: you check the output (gerbview is in kicad, gerbv is from another EDA suite)
email/web: you ship the files and wait for the PCB to be delivered

Finally, mounting
You can buy components and turn on the soldering iron
Or, at extra cost, you can buy, pack and re-ship for mounting (with BOM and POS)
Or, at even more cost, your manufacturer does everything for you
You must be most precise with your BOM file