I should take a moment to clarify that I'm not affiliated with Microsoft or any of the links I'm posting here in any way. I make a living building web sites and as mentioned above, many of them leverage SharePoint. I find it a pretty easy way to build intranet solutions for my customers.
Assuming you've got your SharePoint site created within SharePoint Online, creating a "List" is pretty simple. Here's an article from Microsoft explaining how to do so: https://support.microsoft.com/en-us/office/create-a-list-0d3.... Think of a list as a database table. From there you can create various types of columns including "look-up" columns, which are basically joining to another table on the same site with a foreign key.
I think from there you could dive into the Power Platform, but there's so much out there, I think this would be a good start.
I've always thought SharePoint was terribly slow, but that was just from experience interacting with the web interface itself. Does the API perform well? Or is there that same inherent latency to put up with?
And do you have any general tips/ideas/learnings to share about using SharePoint in this way? I definitely want to give this a try.