This should get you started:

import requests

# Fetch post from Reddit API
def fetch_reddit_post():
    reddit_url = "https://www.reddit.com/r/{subreddit}/comments/{post_id}.json"
    subreddit = "python"  # Replace with the desired subreddit
    post_id = "abcdef"  # Replace with the desired post ID

    url = reddit_url.format(subreddit=subreddit, post_id=post_id)
    headers = {"User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.3"}

    response = requests.get(url, headers=headers)
    if response.status_code == 200:
        post_data = response.json()
        return post_data
    else:
        print("Failed to fetch post from Reddit API")
        return None

# Post to Lemmy API
def post_to_lemmy(post_data):
    lemmy_url = "https://lemmy.ml/api/{version}/post"
    version = "v3"  # Replace with the desired Lemmy API version

    url = lemmy_url.format(version=version)
    headers = {"Content-Type": "application/json"}

    # Extract necessary information from the Reddit post
    title = post_data[0]["data"]["children"][0]["data"]["title"]
    content = post_data[0]["data"]["children"][0]["data"]["selftext"]

    # Create payload for Lemmy API
    payload = {
        "title": title,
        "body": content,
        "community_id": "abcdef",  # Replace with the desired Lemmy community ID
        "auth": "your_auth_token"  # Replace with your Lemmy API authentication token
    }

    response = requests.post(url, json=payload, headers=headers)
    if response.status_code == 200:
        print("Post successfully created on Lemmy")
    else:
        print("Failed to post to Lemmy API")

# Fetch post from Reddit API
post_data = fetch_reddit_post()

if post_data:
    # Post to Lemmy API
    post_to_lemmy(post_data)

PRAW is the Python wrapper for Reddit's API, and searching github for "lemmy api python" revealed some results, so you'd use one API to download a post, and the other to upload it.

There's existing bots that do this though, and they all seem to just fill feeds with indiscriminate, un-replied to, spam

Why does everybody wants to use python 😔

Honestly this should be very straight forward. If your coming and asking this without doing any sort of work on your own does not bode well for you