However I also become liable for paying interest on that 90k, e.g. with total of 30k that I need to pay off in 20 years.
That's only true if you take the whole 20 years to pay it off.
What's really happening is every month (or, more often) they are calculating how much money you actually owe them in that instant, and charging you interest for the use of that money.
Your credit card does exactly the same thing, you know.
People don't like uncertainty, and so they projected that if you make only the minimum payment on the mortgage every month for the full term, then interest will amount to that much -- in fact, this projection must be done to correctly compute the monthly minimum payment. The assumption is that you want all payments to be equal, and have the last payment finish with a balance of $0.00. That's actually a pretty tricky math problem.
It is a plan. You don't have to follow it. You can exceed minimum payments.
This makes my overall debt be 120k
No. That's only true if the mortgage runs to term and you only make minimum payments.
Your credit card has a very similar statement as well. However, with the credit card you don't plan to pay minimum payments forever, now do you? Obviously, if you make more than minimum payments and pay the card off, then that projected interest never happens. So you disregard that number.
Well... same here.
Selling the house instantly pays off the mortgage, which means they stop charging interest. So the 18 years of interest payments never happen.
What really happens.
I fired up a random amortization calc and tweaked it until I matched your facts: $90k/20 years/$30k interest. According it you've been hacking down the principal of the loan about $280 a month. After 2 years the loan principal is down to $83,000.
So. You sell for $110,000. The buyer writes a $110,000 check and gives it to the escrow company. The escrow company gives an $83,000 check to the mortgage. You are out of the mortgage free and clear. They also make out a $27,000 check to you, because that is the rest of the money.
Oh, wait. In reality, the escrow company also takes care of a bunch of items that the seller is responsible for paying. That includes closing costs, Realtor commission, title insurance, blah, blah, etc. There's a zillion of em. So your check will be somewhat less than $27,000, but certainly fine money. You then flip that as the down payment on your next home. (or to pay off the bridge loan you used to get the down payment you already made).
You really don't know much about money, though. Learn more.
The nature of this questions has exposed your inexperience with personal finance. A lot of people have "money skills" programmed into them by family, too, and that leads people to "know" stuff about money that just ain't so.
But in reality, knowledge is power. Knowledge is freedom. I am modestly skilled at money, and that means I work when I want to. I am sitting out the lockdown doing exactly what I want to do. Money doesn't worry me or scare me.
Choose to make it a thing to raise your skills and understanding of money. Start reading/following Suze Orman or Dave Ramsey. Read John Bogle's book "Common sense on mutual funds". Let go of what you know that just ain't so. Make yourself an expert. You'll have a lot more control over your life!