The Pentagon considers eight projects part of its nuclear modernization budget: new ICBMs, cruise missiles, updates to submarine-launched missiles, a new stealth bomber, new strategic submarines, updates to existing gravity bombs, modifications to the F-35 fighter jet that allow it to drop nuclear weapons, and updates to the existing Cold War nuclear command-and-control network. Since 2013, Congress has approved about $67 billion for those, according to Forecast International, a data analysis firm owned by Defense One’s parent company. In fiscal 2021, the funding totaled $13 billion, or about 1.8 percent of the Pentagon’s $704 billion budget. 

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A Rough Breakdown of the Trillion Dollar Cost of Modernization:

1/3 NNSA

The National Nuclear Security Administration is a semi-autonomous agency within the Department of Energy

Warhead Production

• Warhead "Life Extension Programs"
  • Includes upgrades to B61, W80, W76
    
  • New plutonium pit production 
• Nuclear Weapons Labs
  • Los Alamos
  • Lawrence Livermore
  • Sandia 
• Nuclear Weapons Production Plants
  • Pantex
  • Y-12
  • KCP 
• Other Facilities, infrastructure
  • Office of Secure Transportation

2/3 Department of Defense

Warhead Delivery Systems

• Land-Based ICBMs
  • 642 (400 to deploy) new ICBMs 
• SSBNs: Ballistic Missile Submarines
  • 12 new Columbia Class SSBNs
• Bombers & Air-Launched Cruise Missiles
  • Upgrades to the B-1, the B-2, and the B-52
    
  • LRSB: 100 new B-21 strategic bombers
    
  • LRSO: 1000+ nuclear-capable cruise missiles

Warhead Production and 'Life Extension Programs'

• The W76 was chosen as a SSBN (submarine-launched ICBM) warhead.
• The W80-4 is to be the warhead for the next-gen nuclear cruise missile (LRSO).
• The B61-12 upgrade with new Tail-kit.
• Plutonium pit production.

Note that the NNSA (National Nuclear Security Administration) runs all weapons related activities under the DOE. Environmental cleanup of wastes from nuclear weapons production is under the jurisdiction of the DOE without NNSA involvement. While the NNSA budget is set to increase, DOE's is set to decrease.

Warhead Delivery Systems

• New ICBMs (aka GBSD, or Ground-Based-Strategic-Deterrent): This is a contested element of the planned triad modernization. William Perry and others have argued that land-based ICBMs are the one way a computer glitch could end up triggering total nuclear war, and should therefore be eliminated in favor of bombers and SSBNs. Boeing, Northrup Grumman, and Lockheed have submitted bids for the roughly $70 billion job.
• SSBNs are to be designed and built by Electric Boat in Connecticut. Plans call to build 12, at a cost $5 billion apiece, and no-one knows where those funds will come from. There's been talk of a supplemental Sea-Borne Deterrence Fund; but Air Force wants a similar budgetary trick for its nuclear systems.
• Bombers: Northrup Grumman is prime contractor on the new B-21 as well as the B-2 upgrade. Boeing is contractor for B1 and B52 upgrades.
• Long-Range Stand-Off cruise missiles: This is the other most contested of the new systems planned, with many experts calling it dangerous and unnecessary. Air Force has issued a call for bids for 1000+ missiles. Boeing, Raytheon, Northrop Grumman, and Lockheed Martin are expected to submit bids.