1. Does these apply to every transactions?
It only applies to coinbase transactions.
Bitcoin received in other transactions can be spent once it's received (even if the parent is still unconfirmed).
2. If no, please explain the phenomenon behind it?
There's always the chance that a block is orphaned*.
The rule prevents miners from spending the bitcoin that has been generated in a block that may be orphaned.
*An orphan block is a block which has been mined successfully, but it's not accepted due to not being in the longest chain.