TL;DR: bollards and concrete blocks are good at blocking vehicles from driving through, but they are ugly, so you hide them in the form of benches, flowerpots, bicycle racks and other functional things, so people don't notice them. Against explosives you ensure that trashcans are made from non-fragmenting material and try to 'open up' subway stations and other areas with lots of people, so that the amount of chokepoints is reduced, spreading out the people in the process.

https://en.wikipedia.org/wiki/Defence_in_depth

The big thing is standoff; keeping trucks away from buildings.

https://en.wikipedia.org/wiki/Pentagon_Renovation_Program#Remote_Delivery_Facility

The Remote Delivery Facility (RDF) is a new 250,000-square-foot (23,000 m2) shipping and receiving facility adjoining the Pentagon. The RDF significantly improves the physical security of the Pentagon by providing a secure consolidated location for receiving and screening thousands of items shipped to the building each day.

The Remote Delivery facility contains 38 loading docks, and receives an average of 250 trucks per day. The roof is landscaped to create a park-like atmosphere, and is the first major project to utilize a design-build delivery system. The Pentagon helipad was relocated from the site of the Pentagon Memorial to the roof of the RDF.

If you're receiving anything that might be compromised, you have it away from the main building and do your search there.