Can you please explain how taking equity from your house works?

For example: If I had a mortgage for £100,000 and my house was worth £150,00, would i be table to take £20,000 equity out of my house?

If so how does it work, would i then have to pay for that £20,000 on top of my mortgage payments?

I have not got a clue how it works, any advice would be great

Thanks
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