for the [ɪ] vowel, the jaw drops more, so the tongue isn’t as close to the roof of the mouth. There’s a relaxed jaw drop. The tip of the tongue stays forward, lightly touching the back of the bottom front teeth. The top, front part of the tongue arches up towards the roof of the mouth.
Let’s compare the stressed [ɪ] in ‘fix’, on the top, to the unstressed [ɪ] in ‘office’. Notice that the jaw is more dropped for the stressed syllable. For the unstressed [ɪ], the jaw is less dropped. This is typical. Unstressed vowels are shorter, so there isn’t as much time to make the full mouth position.