For Android, there are plenty of local AI clients such as ChatterUI, PocketPal etc. Just download a suitable gguf model from huggingface and use it. If you have 8GB+ RAM, you can easily run 3B models.
Edit: Try to find iMatrix quantized gguf models. They preserve better quality in in smaller size and runs bit faster.
I use ChatterUI (available on Github) because I found it faster and smoother than the rest of the clients available for Android.
Yes, the gguf models on Huggingface are totally safe. They will run locally in your phone. You don’t need Internet connection to run that model. So, it is safe. No data will ever leave your device.
Edit: There is only one client available on f-droid: Maid But it never worked in my device, unfortunately.