Why I can't find any example of using

You can follow this example of Hugginface: https://docs.llamaindex.ai/en/stable/examples/llm/huggingface/

replace the llm with llama3.1 and it should work fine

Same goes for Groq:https://docs.llamaindex.ai/en/stable/examples/llm/groq/

But also, there is an example with Ollama
https://docs.llamaindex.ai/en/stable/examples/llm/ollama/