You can see posts by people on other instances and search for them, but if the account is really new or if it hasn't posted much then it may not be very visible to other servers.
If you follow someone, then they immediately become visible to everyone on your server, and you will see all their posts from that moment onwards.
If you can't find someone by searching, you can use their "account address" to make their account appear: https://fedi.tips/whats-my-accounts-address-how-do-i-follow-other-peoples-addresses-how-can-people-follow-my-address/
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