Is it okay if I just lie down and don’t concentrate on any breathing or mantra? Is that meditation too?

In my opinion it is a form of meditation if you lie down with eyes closed and just focus on listening to the sounds in your environment or laying down and thinking about what you might want/need to do next… these would both be a form of meditation if they cause you to slow your mind/body down out of a state of stress and into a state of focus/clarity/calmness (or even relaxation). In these cases you would still be centering yourself, but without using a specific mantra or breathing pattern (because your body would fix your breathing to a better pattern as you calmed/relaxed).
I actually do this type of meditation quite frequently (daily or sometimes multiple times a day)… I’ve tried mantras and breathing techniques but for me it makes me stress about if I’m doing it right which is counterproductive to the purpose of the meditation.