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. Then you have items you purchase to up your base defense bonus. 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.