This project is no longer supported, discord.js v14 has been released and I'm out of using discord.js v13. Here's the new project: https://github.com/TFAGaming/DiscordJS-V14-ModMail-Bot

REPLIT: Go to secrets, set in the key case TOKEN and in value case your bot token. (To protect your bot token from attackers)
VISUAL STUDIO CODE: Go to config.json and set your bot token in the variable called TOKEN.

Set your server id in config.json, you can also set a category channel in config.json, but not required, because the system when detects the CATEROGY_ID is undefined or 0, then the bot will automatically create a category called "Mod Mail" if you restarted your bot.

If you have created your own category, set the category id in config.json, and delete the created cathegory by the bot (called "Mod Mail") so the bot won't send the mails throught the created cathegory, only in yours.

No, quick.db v7 is not like v9. The version 9 is better than v7 because there are new methods: .deleteAll() and .pull(). Documentation is linked here.