There's more than one Scouse accent. Old Scouse which is South Liverpool (Woolton/Garston/Calderstones/Aigburth etc.). South Liverpool is shared with over the water, Birkenhead/Tranmere etc.
Scouse which is more North Liverpool, Bootle/Walton-ish, think Jamie Carragher. Then round Huyton/Kirby/Prescot way it gets a bit hyper-hard Scouse, like Stephen Gerrard. Then there are mad variants like Skelmersdale/St Helens/Warrington/Runcorn (the Scouse contingent).John Bishop springs to mind.
People were moved out to those places, and others, in the mass destruction/rebuild years after the Second World War and on into the 50's/60's so working class Town accents were transplanted and changed and grew by themselves.
The thing about the kids is they are going for a hyper-hard exaggerated Scouse. But then the 'road men' do that with accents all over, wherever they are.
They are not uniformly spoken according to area either but are all mixed in along with Lancashire/Welsh/Norwegian/German/Irish/Chinese/India/wherever. It depends where your family is from/what school you went to/level of high faluntin'ness etc.
I don't know what accent Martin Freeman's doing, that's a mix of several and completely barmy.
Everyone thinks their own accent is the One, True Scouse though so what do I know 😬