What is "generate" as a bitcoin transaction type? Why are "generate" transactions labeled as "immature" for 100 blocks? Is this type of Bitcoin transaction spendable?
"Generate" transactions are coinbase transactions. These are a special type of transaction that is included in a block by a miner. They can not exist outside of a block. They generate new coins to an address configured by the miner. Those coins can not be spent until they have 100 confirmations. The reason for this is to prevent a messy scenario in which a block becomes disconnected or is no longer in the most-work valid chain, meaning the newly generated coins do not exist on the ledger any more.