The Xiaomi 15 does not support expanded storage via a MicroSD card. This means that you cannot add additional storage capacity to the phone beyond its built-in storage options.
However, the Xiaomi 15 comes in the storage configurations of 256GB and 512GB, which should be sufficient for most needs, offering ample space for APPs, photos, videos, and other media.
If you need more storage, cloud services, such as Xiaomi Cloud and Google backups, can serve as alternatives to ensure that important data is safely stored and accessible.