Sidebar after the content on mobile

Hi.
I wrote to some other issue as well but it says it is closed.

I want to display the left side bar after the content, on mobile version, but without changing the side bar to the right.
is there an easy copy paste code way to display the left sidebar after the main content? With code?
Can you help me with that? Where should i dig in, single.Php, index php and page. Php or does it need to change some css file as well?
it says in a post that ; if you want it below main contetn you should use right side bar? But i dont want to see right sidebar on desktop.
Thank you

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