Think of it like email. Anyone who knows your email address can send you a message, but you're the only one with the password to unlock the account and read the emails or send emails to other people from that account.

Bitcoin is similar. The public address is like your email address. Anyone who knows it and wants to send you bitcoin can send it to your public address. You would then have control over any bitcoin in that address to spend at a later time using your private key.

For example, I buy bitcoin from an exchange and decide to withdraw to self custody. I tell the exchange the public address to send the bitcoin to, they then broadcast a transaction to the network which gets included in the blockchain, giving me ownership of the bitcoin I bought from them.