You can also download custom Civitai models into the app. Go to Civitai and click the green arrow to see if it's compatible before downloading. The best option to download is the safetensors format +pruned if possible. Pruned uses less disk space, and the only downside is that you can't use it to train new models. Model training requires a 12GB GPU so you aren't going to be able to train a new model on an iphone anyway.