Re: Creating an "If a file is opened" trigger
Hi @myNameIsA-aron ,
Flow doesn’t have a trigger for “When a file is opened”. You may try if the following two ways will be a workaround for you.
First, please consider creating a Custom connector, by describing the API you want to connect to, the connector understands the API’s operation and data structure.
More details about Custom connector:
https://docs.microsoft.com/en-us/connectors/custom-connectors/define-blank
Second way is combing with PowerApps. If you are also familiar with PowerApps, you could add a button with Launch function to open the file. Then a flow with PowerApps button trigger to collect data from PowerApps to save to SharePoint list.
I have made the following test for your reference.
Set up the flow likes below:

Add the flow on the button, and set the button’s OnSelect property with the following statement. Then once the current user clicks the button to open the file, his info will be recorded into SharePoint list as well.
Launch("https://xxxxx.sharepoint.com/:x:/r/Test4Testing/_layouts/15/Doc.aspx?sourcedoc=%7Ba432e102-681e-47bb-a0ec-71fe2fe37ec2%7D&action=default&uid=%7BA432E102-681E-47BB-A0EC-71FE2FE37EC2%7D&ListItemId=239&ListId=%7BF8790D59-D66E-4920-8B7B-2D186DE300B8%7D&odsp=1&env=prod");'PowerApp->Createitem'.Run(User().Email)

Best regards,
Mabel