How do interest only mortgages work?

I was under the impression that you don’t own the house at the end of the term so would you have any equity when coming to sell the property?

I just read this post and, ignoring the cashing in pension to pay off debt, their plan is to sell their home which is part repayment and part interest only. Would they only receive money for the part of the property on repayment mortgage?

I have a friend with multiple properties on interest only too and they say it is their pension plan. I guess that makes sense though as income from rental properties?