You can see how there is still equipment required.
Forearms are quite difficult to do without equipment. Unless you include something that could be easily procured at home.
No you can easily do them at home by clasping your hands together and making your arms work against each other. It just isn’t as efficient as compound exercises (e.g. pull-ups that also exercise your back).
Decline push-ups also work on your forearms.
That sounds very inefficient. Here’s something that gets good results quickly. Tie a string/top to a filled 2L bottle. Attach the other end of the rope to the center of a short wooden pole. Wind up the bottle. Wind down the bottle. Repeat ad infinitum. Quick and easy.
Forearms are also one of the main muscles (groups?) targeted by rock climbing. Depending on where you live, you could likely find some boulders with very low traverses, which would require exactly no equipment (though climbing shoes would help immensely)
The inverted rows can be done hanging from a table or desk.
On another note: technically, for forearms you'd want to have your hands facing away from you when doing a chin-up. Hands facing towards you is better for biceps.