A bitcoin is worth whatever someone else is willing to give you for it…that could be dollars, euros, yen, yuan, gold, silver, cars, pizza. Dollars are typically used as the reference point for comparison simply because dollars are currently the most commonly used medium of exchange. But Bitcoin’s value is not dependent on the existence of the dollar, and it will continue to be valuable if/when the dollar fails.
The original author of the Bitcoin protocol (who went by the pseudonym Satoshi Nakamoto but whose real identity is unknown) programmed a hard limit of 21 million into the protocol. That supply limit could theoretically be changed if a majority of the Bitcoin community decided to “fork” Bitcoin by adopting a change to the limit and beginning to run a revised protocol. Bitcoin has survived significant forks in the past (e.g., Bitcoin Cash circa 2017). IMO changing the 21 million limit would not get enough support from the Bitcoin community to cause a significant fork.
If you want to truly “own” bitcoin (i.e., have sole control and custody), open an account with a reputable online exchange (e.g., Coinbase, Gemini, Kraken), buy some, then transfer it to your private wallet. If you just want to invest in Bitcoin (i.e, get financial exposure but not necessarily own it in full), you could do as I described, or buy through an exchange and leave it in the custody of the exchange, or buy shares of an ETF, or buy shares of a mining company.