1st hard drive - will be either the old mechanical hdd or a standard sata ssd (solid state drive). Speeds up to 550mbs read and write for an ssd, much slower for an hdd.
M2 ssd - a smaller format of ssd, basically without the case and connects via a dedicated m2 port. You can see some increased read/write speeds on some of the more expensive drives.
Pci-e ssd - fastest and most expensive drives (newest format). Vastly improved read write speeds over standard sata ssd's. Very useful for number crunching, large datasets, coding of large programs. Not really beneficial for everyday user.
If selecting m2 or pci-e drive, once accepting your build it will ask which drive you want the os on (if you have multiple drives).
Are the M2 drives different on laptop and desktop? I thought M2 could be either SSD or PCI-e, rather than labelling them separately? The one I was looking at was the NVMe Samsung SM951 drive. Or are there 3 variants? M2 SSD, M2 PCI-e and PCI-e due to the form factor on the desktop?