Journalists fall into two categories:
1. Staff writers: These are employees of the magazine. Those who work for Heat and OK will normally be very experienced journalists who've started 'at the bottom' (e.g. writing reports of flower shows, for minimal pay, for a local newspaper) and worked their way through the system to get to their present position. It's unlikely that their jobs were ever advertised as they were probably 'head-hunted' from other journals by their new editor.
2. Freelance journalists: These are writers who are not directly employed by the magazines they write for. In the early stages of their career, they have to write their articles, then send them to magazines in the hope that they'll get published. (i.e. they can spend many hours writing articles for which, if they're rejected, they will get paid nothing). If they become known and trusted by a particular magazine they might start to receive 'commissions'. (i.e. the magazine will agree, in advance, to buy an article which they require).
The writers who report on the celebrity parties, etc, are there for one of the following reasons:
1. The host has invited the magazine to send someone along. This might be a staff writer or it could be a freelance to whom they've given a commission. Either way, it will almost certainly be an extremely experienced journalist.
2. They're a freelance journalist whom the party host trusts to provide some positive coverage of the event. Elton John certainly won't be inviting some unknown writer to cover one of his parties. So, once again, the writer will be very experienced.