The collectibles are part of a meta feature. In an enemy or neutral province a crate is dropped. It will never appear in your ally’s province.
The crates only appear in 22 players, 100 players and pacific maps. When you create a new game you can enable or disable the crate feature.
You have a certain amount of time to conquer this province and collect the crate.
In the top righthand corner a symbol with a timer appears showing you how much time is left.
The crate will grant you either resources or blueprints.
Collecting a certain amount of blueprints of the same type unlocks a new unit tech level.
The progress of unlocking blueprints is visible in the user profile.
The amount of blueprints needed varies per unit. Weaker units are easier to get, stronger units need more blueprints.
When you have collected enough blueprints, you can research the new unit. But this is only possible in an already advanced game. The new units are pretty strong, so you will find them a useful addition to late games.