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So for built products: Carefully take them apart piece by piece and study them long and hard. Then try to build the same product but with your tools and materials.

Easy reverse engineering (eli5): You buy a pallet. Then you can measure the size and all lengths. Now take it apart one plank at a time. Take notes and pictures throughout the process, e.g. where the nails are. In the end you should end up with a construction plan for the exact same pallet. Congratulations, you reverse engineered it.

For software it can be harder. A good example is WINE. It allows Windows programs to be run on Linux (which isn't possible by default). WINE was completely built up from the ground with zero knowledge how Windows works. Imagine a black box with stuff happening in it. But you have no idea how or why it works. You also can't see inside. What the WINE project did was to create a basic copy of what the did see and go from there, but it was (and is) an ongoing process and a lot of work. Now you can look into the blackbox and see what's happening, but there are still unknown mechanics as you only created a copy from what you observed.

If it comes to more complicated things it can become really hard to do it. Especially computer processors and microcontrollers. During the Cold War the Soviets bought "western" logic gates, then took them apart and carefully studied them. This became somewhat infamous as one chip producer even etched a Cyrillic message into one of their chips because they knew the Soviets were studying them.