Bitcoin mining process

Can a person please explain these fuzzy understanding? I am trying to put in force my right demo application of blockchain, but now I discover that many concepts aren’t that clear…

1- As I understand, mining is achieved on blocks in bitcoin. So if someone upload a transaction to a legitimate block, does this mean that the block has to be re-mined?

2- finally who decide to create a brand new block? Is there some conditions on the scale of the block or is it unconditional unlimited process?

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