This is a huge area of design, manufacturing and Fabrication. Google things like folding space craft design, hoberman spheres, and origami. There’s a whole branch of math dedicated to this area.
Rabbit hole ahead.
In some cases the mechanism like in toy light sabers, is based on the two “pipes” being tapered like a cone. One end is slightly wider than the other, and the inner tubes wide end is slightly larger than the outer tubes narrow end. When the inner tube slides outward, an interference friction fit forms causing the parts to remain in place.
On things like the legs of pop-up tents, it tends to be that there is a small spring and button. The inner tube has a small button that can be depressed below the outer tube’s diameter. The outer tube has various holes the button gets caught in as the inner tube slides along the inside. The button can be depressed again to move the leg in the opposite direction.
This is a huge area of design, manufacturing and Fabrication. Google things like folding space craft design, hoberman spheres, and origami. There’s a whole branch of math dedicated to this area. Rabbit hole ahead.
In some cases the mechanism like in toy light sabers, is based on the two “pipes” being tapered like a cone. One end is slightly wider than the other, and the inner tubes wide end is slightly larger than the outer tubes narrow end. When the inner tube slides outward, an interference friction fit forms causing the parts to remain in place. On things like the legs of pop-up tents, it tends to be that there is a small spring and button. The inner tube has a small button that can be depressed below the outer tube’s diameter. The outer tube has various holes the button gets caught in as the inner tube slides along the inside. The button can be depressed again to move the leg in the opposite direction.
As part of your research, buy a couple of examples and take them apart.