Things to keep in mind when writing stuff involving (U.S.) federal offices/federal employees/federal contractors:
You need a badge to get in. If you don't have a badge you will at minimum have to go through a metal detector and have your stuff be x-rayed and sign in and then have an escort. You might get a guest lanyard, a temporary ID badge, or a wristband. At maximum you just won't get in.
You can potentially get in with a badge for a different agency, but you will still likely need to sign in and be let in manually, because your badge won't have building access to swipe you in past security
It can take a long time to get a badge, and it's way worse for contractors. For contractors it can take anywhere from days to over a year to get badged.
Working for the federal government doesn't necessarily mean you have a security clearance. A lot of civilian agencies just require a public trust, which generally involves much lower requirements and a much less invasive background check but can involve the equivalent of a Secret clearance background check (namely if you have a law-enforcement sensitive public trust).
You need to get fingerprinted, which may happen at the actual badging office or at a random contracted fingerprinting place. Basically all fingerprinting is digital at this point.
You need to get fingerprinted even if you have already been fingerprinted/badged elsewhere. They generally don't talk to each other.
Having a clearance can make it a bit easier/faster to get a public trust elsewhere due to reciprocity, but it can still take a while.
Federal buildings are often set pretty far back from the road and/or they have barricades in front of them.
It's not uncommon in some agencies to see mix of people in uniform and people not in uniform. Not all uniformed services are military. Public Health Service Commissioned Corps and NOAA Commissioned Officer Corps are both non-armed uniformed services.
Most federal employees operate under the General Service (GS) pay scale, and people will sometimes be referred to as a GS-X (e.g., GS-13, GS-14). This is an easy reference of relative position.
Some federal employees may operate under other pay scales, such as the Federal Wage System for blue-collar workers. High-ranking federal employees may be under the Senior Executive Service (SES) which is above the GS scale. Other agencies (e.g., the SEC) use their own pay scales.