There's some natural fortification you'll get from any unit (except maybe UHVs?). For static, the fort level will go from x1 when deployed, up to x3 after 90 days. You need construction vehicles in the hierarchy to get higher than that, but if you have them, they'll start fortifying things. I had two construction vehicles (VEH, 2x CON) in a regiment, and within a few years they'd taken my 7200 tons of basic infantrymen from x3.00 up to x4.28 fortification level. (They pick one unit at a time to fortify, it seems.)
It happens automatically, but it's pretty slow.