When pronouncing an 'l' (alveolar lateral), my tongue stays on alveolar ridge of my mouth (the roof, right behind the teeth) for the entire time. When the 'l' sound ends, my tongue leaves the roof of my mouth, but it never "strikes the bottom" of my mouth. If there's a vowel next, it moves to a neutral position for a vowel. If the 'l' was the end of the speech, it just kind of sits in place for a second after the noise stops. If it's followed by another consonant, it moves for that articulation (e.g. when saying "all the sounds", it moves directly from the alveolar ridge to the teeth for the dental fricative).
When pronouncing an 'r' (alveolar approximant), I, and I think most English speakers, roll the tongue backwards to some degree (although not enough that the point of articulation is the bottom of the tongue). I.e. the tongue does not "stay at the bottom". Yes, it is certainly possible to pronounce an English 'r' with the tip of your tongue held down (unlike 'l'), but I think most English speakers would find this "impossible" at first, and then with a minute of practice would realize that it was possible but difficult. In English, these two articulations are perceived as the same phoneme; I assume that some language somewhere differentiates between them. I notice that often when I speak (English) to natives of India (whose first languages I have not identified), I get the sense that they have a richer complex of 'r' noises than I do.
In summary, "l" does not involve "striking the bottom [of the mouth]", and "r" typically does not involve the tongue staying at the bottom of the mouth, although it can.