It's been a while, but ...
It's been 3 or 4 decades since I was involved in government contracting. But I doubt things have changed much. Every contract award is a bit different, but the general process is sort of generic.
The Gubmint comes up with a Request For Proposal (RFP) that defines the general parameters of the contract. Time span, The sort of price they have in mind. Some things they want done. Completion date. An incredibly lengthy list of laws and specifications that must be complied with. How specific the RFP is, is a function of what is to be done. If they are looking for 300 Tomahawk missiles, your proposal better specify devices the same size and range with identical or near identical capability, with the same interfaces. And they'll probably specify a test regime. In this case they probably specified workloads, data volumes, security constraints, reliability, perhaps acceptable OSes and programming languages, page after page after page ... lots of specificity.
You as a contractor will have to "qualify" yourself. You'll need to come up with a plausible story about how your operation has adequate staff, experience, etc,etc,etc. to do the job
Once you've qualified yourself, you then produce a lengthy work of fiction called a Proposal that explains in detail how you will do the job in the specified time period while fully complying with every relevant law and constraint. And how much it will cost. This is where it gets trickier. Different bidders will likely have different approaches, skill sets, hardware preferences, etc, etc, etc. If the contact is for 50,000 pairs of combat boots, the proposals may be relatively easy to compare and the contract will likely be awarded to the low bidder(s). For complex technical products, likely no one is completely sure what is being offered, when, and for how much. But tradition demands that everyone ignore that.
Anyway, the government may well come back with a few (or a lot or) questions. Revisions to the proposal may be allowed if it becomes clear that there is a genuine misunderstanding about what is desired and some of that is the government's fault. Eventually, the contract is (usually) awarded assuming that there is at least one (marginally) acceptable proposal.
Then comes the lawsuits and pontificating by guys with expensive haircuts and even more expensive suite. That's the phase we seem to be in now.