How exactly does the procurement process work

I’ve often read fighter procurement programs where countries insist on government to government instead of traditional agreements with a company. What exactly does that mean ? What are the benefits of such a deal ? For example in the U.S does Congress have to approve every export sale down to Humvees and spare parts or do these small items sail through ? How many potential deals get blocked or modified due to security concerns ?

Any description or links to websites detailing how the procurement process works from start from to finish would be greatly appreciated 🙂