Any idea how this plugin appended custom fields to post content?

Now that TDO seems to be abandonded I have moved on to a manual method of accepting form submissions. I’m currently using a contact form but have created (and hope to use on my live site soon) a manual front-end-submission form with the thanks to some tutorials (see my post here if anyone is interested on how to do that themselves: https://www.ads-software.com/support/topic/how-to-use-nonce-in-front-end-post-submission-form?replies=1)

My question now though is – how did TDO append custom fields to the content? This is the only feature I miss! I cannot find any information on how to do this. I’m going to take a look at the code from the old TDO and see if I can figure it out but it may be a little beyond my skill level at the moment.

I know how to display custom fields by calling them in a template, but how were they inserted in to the actual post content?

Any pointers from anyone would be greatly appreciated ??

Cheers

https://www.ads-software.com/extend/plugins/tdo-mini-forms/