The content will not be lost, because each federated instance keeps its own copy.

I keep hearing that it doesn’t matter which instance you join because of federation.

It doesn't matter as long as you use one of the top 10-15 instances. That's the rule of thumb.

If people want to get more exotic, they should pay attention to that the instance follows the Mastodon Covenant (https://joinmastodon.org/covenant):

1. Active moderation against racism, sexism, homophobia and transphobia

Users must have the confidence that they are joining a safe space, free from white supremacy, anti-semitism and transphobia of other platforms.

2. Daily backups

It is important for users to have the confidence that a trip over the power cable or a rogue bit flip will not erase all of their data. Having a backup strategy is a basic necessity of providing a public service.

3. At least one other person with emergency access to the server infrastructure

Various circumstances can prevent the original owner of the Mastodon server from answering technical emergencies. For this reason, more than one person must have that capability.

4. Commitment to give users at least 3 months of advance warning in case of shutting down

Sometimes services shut down, it is the cycle of life. But users must have the confidence that their account will not disappear overnight, so that they have time to export their data and find another server.