Hey @HIREiN,
Form 121 replaces both Form 15G and 15H. We have written a detailed thread on what’s the difference between the new form and the old ones, please find it here: Form 121 vs Form 15G & Form 15H: Nil TDS declaration explained
You should give Form 121 to the entity making the payment (the deductor). This could be your bank, insurance company, or the organisation where you hold bonds or similar investments.
Hope this helps!