A mined block contains mining reward and transactions
Unconfirmed transaction are in the mempool
Delayed or stuck transaction is because of the low fee used for it
Delayed transaction also can be caused by mempool congestion
Use a wallet that support replace-by-fee like Electrum, Sparrow, Bluewallet et cetera. With RBF enabled, you will be able to pump the transaction fee to increase the chance of miners to include your transaction in the mempool into their candidate block.
For fee estimation, these two mempool sites are recommended:
https://mempool.space/https://jochen-hoenicke.de/queue/#BTC,24h,weight for more accuracy.
For lower transaction, use segwit addresses and always consolidate your high input.
Block in btc contains sets of transctions made by users ,
Mined blocks contain the mining reward which is presently 6.25 BTC. They also contain the transaction the miner included which become the confirmed transaction. The miner takes the mining reward and the transaction fees as the incentive.
Now let's go back to our delayed transactions. In relation to Block size, pending transactions might has filled up a block therefore, causing delay in your pending transaction for some time befor it get solved on the block.
Pending transaction can not fill up a block. Pending transaction are in the mempool. Mined block contains the transaction a miner included into his candidate block that later become the mined block.