If your budget won't stretch to off-the-shelf lens kits for phone camera macro work, look around for a used jeweller's loupe in places like charity/thrift shops, car boot/garage sales etc.
Blu-Tack or equivalent will hold the loupe flat against your phone camera's lens. Centre the loupe. Check your viewing screen to minimise any circular "borders" then have some fun experimenting with extreme close-up shots/macro work.
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