It's not some single centralized trusted source. It's a local trusted source. Like a friend or a shop or a website you make payments on that uses the network and has been keeping up to date. Ideally folks should check multiple sources to ensure they agree.
In the same way folks need to figure out which software to download when they join the Bitcoin network.
Also, two other comments regarding weak subjectivity:
When you join the bitcoin blockchain, you need some trusted source to tell you the hash of the correct genesis block.
Also, if you want to follow a shorter fork chain like Ethereum Classic, you also need weak subjectivity to tell you the first block immediately after the fork, otherwise you might be tricked onto the longer malicious fork, Ethereum :P
> When you join the bitcoin blockchain, you need some trusted source to tell you the hash of the correct genesis block.
No, you don’t. There’s nothing special about the Bitcoin genesis block, it’s a block like any other. Whether you follow a chain that builds on top of this block or some other block has no bearing on the security of the system. It contains no keys that get to decide anything later on.
In the same way folks need to figure out which software to download when they join the Bitcoin network.