Amazon Sr Data Scientist interview questions

He seemed like someone who just went through some cool ML/DS terms. Asked about how to fit a logistic regression, PCA, p-value, A/B testing, gradient descent, bias-variance tradeoff etc. In scripting question, he merely asked how to find mode of a list without using any in-built maths function or library.

Q: If there are 10 million variables, how can you reduce the dimensions?