Hello ‘Jemmus’ and welcome to the forum.
Don’t worry. Fasteners aren’t boring. Should I say they’re fastenating. Oops.
Seriously though this parts looks to be 90168-60013, which goes into 90189-06084 (on the EPC). The screw is a very standard part, though the EPC seems to show it’s only used here. Most likely the part number has been updated but the EPC shows it to be a zinc-plated, flange-head, self-tapping 15mm machine screw. The EPC clicks through to the Amayama site with pictures of the part. You can buy them there for £1 each or just buy a bag of fasteners for very little more.
See: https://toyota.epc-data.com/regius_ace/lh119v/114894/body/5854/ (for my van).
If you also need the plastic part these screw into then, again Amayama has pictures of these, which will help you decide what to do.
You might like to take a look at the first post in my ‘show-us-yours’ thread where I describe a few resources that are handy ways to look for all sorts of parts etc.
Hope this helps!!