The title of "PCB Builder" is a bit confusing. At first I thought they were offering assembly but it appears to be only bare boards. With their parts business assembly would seem a natural fit but the parts picker and soldering systems are probably quite expensive.
There are so many PC board assembly houses out there that I don't think Digikey would want to get into that business. Here is the full text of that Digikey email:
PCB Builder
PCB Builder is your newest partner in prototype board manufacturing. Simply upload your Gerber file and get an instant quote for an unpopulated PCB
from several quality board houses including DKRed™, Digi‑Key’s high-quality, low-cost option within PCB Builder for all your prototyping needs.
PCB Builder Offers:
• Boards Up to 6-Layers
• Low Order Minimums
• Quick and Standard Turns | 2 Days – 10 Days
• Multiple Board Provider Options
• Quick or Standard Shipping Options
DKRed | PCBs in Our Classic Digi-Key Red Soldermask:
• 2- and 4-Layer Options
• Low Physical Size Minimum of .5'' x .5"
• $1.50/in² (2‑Layer) | $3/in² (4‑Layer)
• Minimum Purchase of 4 Copies
• Build Time: 10-Day (Standard) or 5-Day (Quick-Turn) Options
• Made in the USA With Free Shipping to US Addresses