Your device must support eSIM and must be unlocked for mobile carrier use. Common compatible devices include iPhone XS/XR and newer, many Samsung Galaxy S/Z models, Google Pixel 3 and newer depending on region, and selected tablets.
On iPhone, check Settings > Mobile Service or Cellular > Add eSIM. On Android, check Settings > Network, SIMs, or Connections for Add eSIM, Add mobile plan, or Download SIM.