I found a THING:
In order to craft a new frame, you must get 4 blueprints for it: Helmet, Chassis, Systems, and the blueprint that puts those 3 pieces together. The last blueprint can be bought in the market for 25k-35k credits.
To acquire the other 3 blueprints, you must kill the boss of a planet multiple times. Each boss drops all 3 parts of a single frame, and different bosses drop different sets. Example: Jackal, boss of Venus, drops the Helmet, Chassis, and Systems for Rhino. Councilor Vay Hek, boss of Earth, drops the Helmet, Chassis, and Systems for Trinity.
The chance for bosses to drop a blueprint is random, so you can get all 3 pieces in 3 runs, or literally take dozens, if not hundreds, of runs to complete the set. If a blueprint drops, it will look like a small red version of an affinity sphere. Be sure to place a waypoint on it if you’re in a group so your teammates can pick it up too.
Once you have the blueprint, you can craft it in your foundry. It takes 12 hours to craft each piece, and 72 hours (3 days) to put all 3 pieces together. Once the crafting is finished, you can claim it and it will be added to your arsenal. You only start with 1 spare frame slot, so make sure you have enough space. You can buy additional slots for 20 platinum each.