Relating to platypus and horizontal gene transfer, I found this, which seems to be not quite what you're talking about: Horizontal transfer of BovB and L1 retrotransposons in eukaryotes. Genome Biology, 19(1). doi:10.1186/s13059-018-1456-7
Regardless, horizontal gene transfer certainly does happen.
If you'd like a curated collection of many papers on horizontal gene transfer in eukaryotes, it comprises much of the evidence presented at https://www.panspermia.org/archindex.htm . Having followed this collection since 2010, I have gotten the sense that horizontal gene transfer to/from eukaryotes is not only common but an important mechanism of evolution.
Yeah most people think evolution is just natural selection and accidental mutations, which is a high schooler understanding's of it. I'm not being derogatory, it's literally what I learned in high school and I don't blame people for not digging further if they haven't taken classes afterwards.
Well I wouldn't normally but because you're so obviously wrong I thought I'd try. First, I can't find any reference to what you claim. The platypus is a mammal, end of, AFAICT. It has kept features from it's long evolutionary path (split off other mammals 165 million years ago) but no evidence of it being from gene-jumping I could find. eg. https://abcnews.go.com/Technology/story?id=4828261&page=1
"This process is called horizontal transfer, differing from the normal parent-offspring transfer, and it's had an enormous impact on mammalian evolution." For example, Professor Adelson says, 25% of the genome of cows and sheep is derived from jumping genes.
https://news.ycombinator.com/newsguidelines.html
(I took a quick look at your account's past comments and didn't see any previous history of this, which is great.)