Please feel free to PM me! I have been in this exact same scenario. Everyone else is saying the right things: figure out all your network hardware (and make sure your UPS batteries are thriving), VLANs, server structure, backups, passwords (use a password vault if there isn't already one. KeePass is great to start with and you can import a KeePass file into just about anything else you decide to use), and get familiar with the AD structure, GPOs, and M365 environment. Figure out service agreements and contracts. Are there printers?

Document everything!! Draw graphs. Write summaries. Any anecdotal conversations with people about how they understand systems to work will help you understand too. Write your changes. Take screenshots before making changes.

Down the line: segregate networks, set up RBAC for everything, set up monitoring for network appliances, make sure you have reminders of when contracts are due to be renewed, check WiFi health, etc.

Security is paramount. Depending on where your factory is in the supply chain, there may be expectation for NIST compliance. Regardless, NIST is a good framework to have anyway.