sd card is external memory card.
There is a sd card holder inside the phone, you can purchase an external TF card to expand the storage space, with TF card you can store videos, photos and other files.
Some card holders need to open the back cover to install the TF card, and some card holder sockets are on the side of the phone, so you can insert the TF card directly.
Android phones have this advantage for ease of use.
Apple phones and iPads do not have sockets to insert SD cards, which is inconvenient to use, and there is a special lightning storage through the USB port, which can be plugged into the USB port and also expand the storage space. Not very convenient.