say I add a few things to the game (new doors / a custom building, that i do not want to be placed (as a multi)).. these are things to go in statics?
Yes, if you added them in a map editor. If you added them in game they are dynamic unless you use an external program to freeze them to the statics file. There are programs that can take your world items (items.txt) and make them static but I have never used any.
Will the things be usable?
No, static items are not usable.
And will updates to the Statics change the minimap?
Yes, they will. The same is true for multis as well.
When you are editing the map and placing objects on it like a wall, those are added to the statics files for the particular map you are editing. Items that you place as statics are not usable. They cannot have a use
script attached to them. They are not movable, deletable or manipulable in any way in game. If they are a light source such as a lit lamppost they will glow and produce the normal light effect when the region light level is dark enough but unlike a dynamic or .created item they cannot be turned off or on by double clicking them. The client will display their name if you single click on a static item. You shouldn't place doors, or any normally usable item, on a map with a map editor (ie. static) unless you want them to be unusable. Items added to a map using a map editor are not part of the multi.mul files.
Multis are similar to statics except for a couple of things. Multis are placed in game, ie. while logged in as a character. Also, some parts of a multi structure are dynamically placed in the world by POL when the multi is created. When you use a house deed and place a multi house it is almost virtually equivalent to a static house in that most of the individual pieces of that multi cannot be manipulated in game. You need to be able to open doors and use a house sign so those have to be dynamically placed. You will notice, if you place a multi, even as a GM you can't .destroy the individual parts of that multi except for the items which are placed as dynamic items. When you place a multi in game the information describing each non-dynamic item (serial, objtype etc.) that make up the multi is saved in the multis.txt file in the \pol\data directory. The information describing each dynamic item needed by a multi is placed in the items.txt file. The doors and other dynamic items that get added after the multi structure is placed can be .destroyed because they are saved as dynamic items. There is a separate command used to destroy multis. On my shard it's .destroymulti. That command removes all items that are associated with that multi including any dynamic items that it required such as doors or signs. Note that it leaves behind any dynamic items a player or GM placed in the multi.
I hope that clarifies the difference between static, multi and dynamic items.