It really depends on what you are planning to add as to how it is addressed.
Like Vanilla beans can simply be soaked in vodka for a few and then added. Coffee beans are usually cold steeped and then the liquid is added. Fruit is a whole other story and it depends on the fruit, fresh, frozen, pureed, etc. All of these things are usually added in a secondary vessel once primary fermentation is complete. The only thing most people will add to a primary is dry hop additions and they can just be thrown in.
You have to remember that once fermentation is complete the alcohol in the beer makes a pretty inhospitable environment for things to grow in many cases.
If you know what you want to add specifically then it is easier to search that specific item