Basically, the Orion/SLS system is your typical hodgepodge of contractors just like the Saturn V and Shuttle were:
SLS core stage: Boeing, based on the Shuttle External Tank and fitted with four RS-25 (shuttle) engines. They have enough shuttle engines left for 3-4 flights and they've been modified and upgraded, and a contract is out for 18 new ones to be manufactured. Second core stage is currently being built.
Boosters: Northrup Grumman (from their acquisition of Orbital ATK), based on Shuttle boosters but extended by one additional segment and non-recoverable. They have enough booster casings left for 8 flights, after which NG will replace them with composite-casing boosters that are being developed for their Omega commercial rocket.
Upper stage: First three flights will use a modified version of the United Launch Alliance Delta IV cryogenic upper stage. Boeing is developing the more powerful Exploration Upper Stage intended to start flying with Artemis 4.
Orion: Lockheed Martin, using a service module provided by the European Space Agency and built by Airbus.
The first interim upper stage from ULA is ready, the first Orion capsule and service module are essentially ready, boosters are shipping to Florida soon. The first core stage is on the B-2 test stand at Stennis for green run testing that they just recently resumed work on after virus shutdowns and is the main hold up at the moment, causing the first flight to slip to 2021 instead of this year.
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