If Bitcoin is going down, do I only lose if I sell my bitcoin? So, if I hold them I won’t lose?
Well basically, think of it as if u had an apple. If you decide at any moment that you want to sell it, you get a specific price for it, which is the current price of an apple.
So if you bought 1 bitcoin when 1 bitcoin was worth 18K USD and sold it at 27K USD you have now a profit of 50% which means that you now have 27K USD as fiat in your hand instead of having 18K (which you had when you bought a single BTC worth 18K back then)
This works the same if you buy it at 27k and sell it at 18k, except, you then lose 33% of your money that you invested in it when you bought it. That means that if you buy 1 BTC at 27K and sell it at 18K because maybe you are scared to lose more, you end up losing 9K because you sold it.
If you don't sell it, you will remain with that 1 BTC the whole time when it's price fluctuates, but it is basically worth more (or less) depending on it's current market price.
Yes, you only really take a loss (or make a gain) in terms of dollars or other fiat currencies when you trade whatever cryptocurrency you're holding back in for fiat.
And yes, it's similar to other assets that can be traded for fiat currency, like stocks, bonds, other fiat currencies, houses, apples, etc. "On paper," as they say, like if you were keeping accounting records to estimate the net worth of all your investments, the current value of that investment has gone down and so the net worth of your investments goes down. But you haven't "realized" the loss until you've sold the asset at its current, lower price. So if you can hold it until the price goes back up again, you can "realize" a gain instead of a loss.