No, that is just what Dell offered configuration.  You can certainly make any storage drive a primary boot drive.  If Dell ships you nvme ssd + 2.5 ssd, you may want to remove the nvme ssd then do a clean install of OS on the 2.5 ssd.  But before you do this remember to switch bios>sata operation to AHCI (Dell default is RAID). having two boot drives can be confusing, so if you decide to keep 2.5 ssd as boot drive, better not to install nvme ssd that has Dell installed OS.
How do I boot optiplex 5070 MFF from an internal 2.5 SSD disk? 2.5-inch SSD is only available as secondary storage, does that mean bios will not use them for booting?    This page says - "  
Boot Optiplex 5070 MFF from internal 2.5 SSD