If you are from a visa exempt country, and would like to visit or transit through Canada by air to another destination, you would need to have an approved eTA. However, if the citizens from visa exempt countries would like to visit Canada through land or sea, they do not need an eTA.
Citizens & Permanent Residents (Green Card Holders) of the U.S. do not require an eTA or a Visitor Visa to visit Canada. They can travel with a valid U.S Passport and stay in Canada for a period up to 6 months.
Extending your Visit:
If you are already in Canada as a Visitor, and need to remain here for longer than 6 months, you must apply for an extension of Visitor Visa while your legal status is still valid. If the Visa Officer is convinced regarding your need to remain in Canada, you will be given an extension of status.