Deleting photos from iPhone

This has been posted before but years go and apologies for asking again but still very unsure if it’s okay to delete photos on an iPhone and how to do it, and then be confident 100% that these photos will remain on my iPad and not suddenly all disappear into the ether from my iPad! I have a business phone I have to hand back but have an iPad and am waiting for a sim in the post for my new iPhone and want to reinstate the pictures back on to my new iPhone when it arrives. Thank you and thanks for responses hopefully.