First ques:where in the blockchain is a newly created transaction stored ? In the blocks that each node stores. will my transaction details reside on every block after block X or only on block X + 1 ? Each transaction only resides in a block. Each block is related with the previous block, so your transaction is throughout all the blockchain.
second Ques: And will my transaction details be the only details on that block, or will that block contain every transaction that happened within that time period ?
It depends on the implementation of Blockchain. For example, Bitcoin blocks store all the transactions that have been sent throughout 10 minutes, because each block is mined every 10 minutes (more or less). Second Blockchain is a distributed system, where all the members are at the same level. So, they get the consensus about what they are going to do, i.e. all the members have to agree about the functions of their Blockchain. For each Blockchain, you can have more than one contract. But I'm going to explain it more simply. The Smart Contract is the code that is installed on all the nodes of the Blockchain, and every request is executed against it. So, every node/member must have the same.
