Are there any instructions on the pose of hand during meditation?

First is the 'jnana mudra' or vitarka or knowledge mudra. Second is the 'dyhana mudra', or meditation or cosmic mudra.

The first is the said to represent the continuous flow of wisdom with the circle of thumb & forefinger, and is said to help with clear perception & gaining knowledge.

The second is the triangle between thumbs & hands represents to represent the Three Jewels, & is said to help provide calmness & tranquility, and help eliminate mental disturbances.

The more & the longer you meditate, in my experience, the more you appreciate the subtleties of position. Keep noticing, keep thinking about why you are on the mat.

Initially, there may be some value in adopting a mudra. For beginning meditators, I think they can help combat sloth and overall mental laxness. I personally used the "zen mudra" for the first year or two of my Rinzai practice. But I think it's important to recognize that they're all ultimately unimportant. As your meditation practice deepens, it's going to seem as if your body has dropped away entirely, merged with your consciousness, or has undergone some other comparable shift that radically alters your physical sense of self. At that point, where your body is in space - to say nothing of your hand position - isn't going to mean a damn thing.

FWIW, I've been sitting with my hands just draped in my lap for the last 14 years to no ill effect. If I had to give any advice, it would be to just pick one that you like. When it ceases to be of use or seems to be getting in the way, feel free to discard it.

These are called mudras. They're not just symbolic; they're functional, in the sense that they have an effect on your meditation practice. The most common mudra that Buddhists use would be the Dhyana mudra.

If you want to see what it does, you could sit with hands on your knees facing downwards, note how it affects your breathing, then flip palms upwards the next day and see how your breathing changes. And then slowly try different postures to see how they make you feel. If they feel the same to you, then you haven't gone deep enough to notice the subtleties.

My advice regarding such things is; if you're a layperson and not totally committed to spending a lot of time meditating, just pick the one that's most comfortable to you. Worrying about the posture would be like a casual jogger worrying about the exact brand of running shoe; it only really matters when you reach the level of an Olympic athelete.

Clean hands are important:

MN66:12.2: He’d see a mendicant sitting in meditation in the cool shade, their hands and feet well washed after eating a delectable meal.
MN66:12.3: He’d think,
MN66:12.4: ‘The ascetic life is so very pleasant! The ascetic life is so very skillful!

But seriously, there's a wide diversity of hand positions throughout Buddhist traditions. For example, my Rinzai Zen teacher instructed us to grasp left thumb with right hand nestled in left hand. See also ruben2020's suggestions in the comments above.

Overall guiding principles would include refraining from dullness, drowsiness, and restlessness.

DN34:2.3.68: It’s when a mendicant has given up sensual desire, ill will, dullness and drowsiness, restlessness and remorse, and doubt.

In terms of positive attributes the Buddha instructs:

DN34:1.6.73: ‘This immersion is peaceful and sublime and tranquil and unified, not held in place by forceful suppression.’

From these clear instructions, one may infer that meditation hand positions should be relaxed without being slack. They should also be engaged without tension.

Notably, the subject of meditation is more important than the form:

DN34:1.6.96: But a meditation subject as a foundation of immersion is properly grasped, attended, borne in mind, and comprehended with wisdom.
DN34:1.6.97: That mendicant feels inspired by the meaning and the teaching in that Dhamma, no matter how a meditation subject as a foundation of immersion is properly grasped, attended, borne in mind, and comprehended with wisdom.
DN34:1.6.98: Feeling inspired, joy springs up. Being joyful, rapture springs up. When the mind is full of rapture, the body becomes tranquil. When the body is tranquil, one feels bliss. And when blissful, the mind becomes immersed.
DN34:1.6.99: This is the fifth opportunity for freedom.

Consult with your teacher for specifics of hand positions used in your chosen tradition.