A larger number of qubits allows us to do effective quantum error correction. The idea is to group multiple physical qubits into one logical qubit, think of it as redundancy.
Genuine question. I've no idea.
Generally, the number of physical qubits scales linearly with the number of logical qubits.
[0] https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.04... [1] https://www.nature.com/articles/s41586-022-04566-8