Your best building to get your information is your Defense Core.
Inside here you will get your base defense bonus (percentage) and your Base Defense.
There is a good article announcement on this in detail here   https://plarium.com/forum/en/soldiers-inc-mobile-warfare/news-and-announcements/38198_defense-core/
However the gist of it is, you have emplacements or buildings such as turrets that are similar to units and build your Base Defense. Â Then you have items you purchase to up your base defense bonus. Â This bonus applies to your units at your base (not in bunker).
So let's say you have a Base Defense of  2000
A bonus of 10%
And you have units with a rating of 2000 defense in your base.
You would use your bonus on the units so  2000 * 10%  = 2200 + your base defense (2000) = 4200 tl defense at that moment.