Your wording is awkward op,
is it legal to extend your contract yes,
is it legally required to extend your contract no absolutely not,
do they have to put it out as a vacancy, no, not legally, only if their own policy dictates they need to,
can they decline to put it out as a vacancy and just extend, yes again, absolutely legally, but if their own policy states they need to advertise and interview for the role at each contract expiry they will need to do that,
so you need to ask what the policy is, I assume you wish your contract extended nd to stay and you need to speak to your supervisor.
in addition, you’re on a temp contract, for a specific time period, they may not wish this to continue and to permanently recruit to replace the original candidate, this is overwhelmingly likely to go to interview in a higher education setting, as many more candidates will apply for a permanent position than a temp one,