They are related, but are not the same.
The words "strategy" and "tactics" are both military in origin. Both words assume that you are fighting an opponent. The opponent is another human being who is intelligent. You are trying to outwit each other. The words "strategy" and "tactics" can be used for military operations. They can be used for games like chess or football. They can be used for business maneuvers against another business who is competing with you.
"Strategy" refers to large-scale plans. It refers to big goals. Military strategy is set at the highest levels of the military or of government. It might take years to achieve. It might involve whole nations or whole armies.
"Tactics" refers to small-scale plans. It refers to things a battlefield officer might do. Tactics are detailed plans. They are little strategies for accomplishing the little parts of the big strategy.
To use a business example, consider Henry Ford and the Ford Motor Company. When he started, cars were expensive. They were built a few at a time. They were built slowly and carefully. Henry Ford had a different strategy. His strategy was to "build a motor car for the great multitude. It will be so low in price that no man will be unable to own one.” To support this strategy, he used tactics to reduce the cost and increase the speed of manufacture. One of them was the assembly line. He give a single simple task to an unskilled worker, to do over and over.
A "plan" is any series of steps to accomplish a goal. The word "plan" does not suggest anyone fighting back at you. Next year we're going to take a road trip on the Blue Ridge Parkway. I am using maps, guidebooks, and websites to plan the trip. It's just a plan. Nobody is trying to stop us.
It's true, though, that we can use the word "strategy" to refer to the big general elements of a plan, as opposed to the details.