Some rollback doubts

>From the docs : Many active transactions can write concurrently to a single
rollback segment--even the same extent of a rollback segment; however, each data block in a rollback segment's extent can contain information for only a single transaction.

1. Now, when oracle writes to the last extent in the cycle, and sees that the first extent has an active transaction, it allocates a new extent. Well, why should it? Cant it write to the first extent?
2. When a new transaction needs to use the rollback segment, and say it gets allocated to a rollback segment with a few other transactions, how does it decide which extent to use from the cycle of extents?

Thanks
Raj

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