Let’s say you had 10 bitcoin you put in a BTC wallet in June of 2017. That balance - 10 coins - is visible on the blockchain, which grows by another block every 10 minutes or so, like a ledger adding another page every 10 minutes. Let’s assume you never spent those coins.
In August 2017, there was a hardfork intended to create a new coin in a live split of BTC, which we now call Bitcoin Cash. This new chain was identical to the Bitcoin chain up until August 1st, 2017, but it was not compatible with the Bitcoin chain, so the chain grew as a new branch of the “family tree.” So BCH was a new coin, but had the same historical ledger as Bitcoin before August 1st.
That BTC wallet address with 10 coins has a corresponding BCH wallet address, and because they have the same history from June, the Bitcoin blockchain says you have 10 coins and the Bitcoin Cash blockchain says you have 10 coins. This is not because more coins were mined - it’s because there are now two different ledger books, and they both say you have 10 in a wallet address that you put there in June.
Then, on October 25th, 2017, Bitcoin Gold copied (forked) the entire Bitcoin blockchain history as the basis for the new coin, BTG (which was launched on November 12.) So, on November 12, a new chain began to grow, with a new coin, but which contained the full history of BTC prior to October 25. That means that in the BTG chain, you have 10 coins which were put in that wallet in June.
At that point, you had 10 BTC, and 10 BCH, and 10 BTG. None of this was because of new coins being mined - all three chains have roughly 17,000,000 coins in existence today. The reason it’s 10 coins of each is simply because they have a shared history - a history in which you had 10 coins before the forks. At the time of the split, 1 coin BTC led to 1 coin of BCH, and at the time of the Bitcoin Gold fork, 1 coin BTC led to 1 coin BTG.
Current prices for the coins are an entirely separate matter. You can say that 1 BTC is worth as much as 219 BTG, but that’s not to say that 1 BTC is the same thing as 219 BTG - you’re not creating BTG coins from BTC coins, they simply have that price ratio. It’s a price ratio, not a coin ratio.
Think of this: salt costs about $0.008 per ounce. Gold (the metal) costs about $1223.45 per ounce.
So we can say that one pound of gold = 2,446,900 pounds of salt, in terms of price… but that doesn’t mean that having one pound of gold is the same thing as having 2.5 million pounds of salt! We’re just talking about a price ratio.