My son was 4 when he knew he was a boy. He confided in a friend.
We didn't even have broadband at the time (mostly because I didn't realise how to use it, but still😁) and was only watching CBeebies and a few other TV shows. I really don't think he learned about being trans from watching Harry Hill sing about Amanda Lamb.
He was raised by a feminist who was a bit dismayed to have to learn about trad 'girly' things in her 30s. When he went to Covent Garden for a ballet audition, I paid a hairdresser to put his hair in a bun. Eventually, his long-haired dad learned to plait, braid, so buns etc but I never did. He was a child who loved pink, dresses, sparkles, unicorns and JoJo Bows, alongside other friends who had no such interest.
I had him wearing his brothers' cast-offs til he was about ten, when my (utterly delighted, because she raised me) mum told me I had to come to terms with him being a 'girly girl'. I loved him but was so, so bad at that aspect - left it to his aunt, gran etc.
He tells me that knowing he was a boy didn't affect or influence what he liked to dress in - and it still doesn't.
By 13-14 he was the sole trans boy in his school. He left as the sole trans boy.
Trans people are born, not made. Clothes do not make them trans, nor does the internet or 'social contagion'. He is simply trans, no one else caught it (!) from him, no one told him what he knew at the age of four.
Not raised by wolves, but by a feminist who had no idea about being trans til he came out to me. He'd known all his life - because it was who he was.
It's not about a feeling, or clothes, or cosplay. It's a deep-rooted knowledge of who you are.
Trans girls are the same. And dear god is that a difficult life at school - trans boys have it marginally easier, in my experience - especially at secondary, when the horrific bullying all trans kids experience becomes sexual harassment and assault.
No child would be trans unless it meant everything to them.
They're a tiny minority. If you want to understand, read their own words.