How can I maintain utmost privacy on both smart phone and pc?

Sorry if this question has already been asked.

Since watching a few documentaries such as "The Great Hack" etc...I am increasingly uncomfortable with big tech (and other nefarious entities) having such easy access to my data.

How can I maintain the utmost privacy on both smart phone and pc? The more detailed answer the better.

Any and all responses are much appreciated.

Thank you!

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