Checkpoints: The Default Model Of Stable Diffusion
Checkpoints have been around for over a year. Originally we kept them in a .ckpt file format, some are in .pickletensors, while the majority nowadays are in a .safetensors format.
You can find Checkpoint files on Civitai (opens in a new tab), although HuggingFace also has some checkpoints like BreakDomainR (opens in a new tab)
Once you download a model, you can put them into the stable-diffusion-webui/models/stable-diffusion folder
Once you load up A1111, these models will show up in a dropdown list in the top left corner.
If you'd like to learn how to train checkpoint models, then I would recommend checking out Olivio Sarikas' video (opens in a new tab).