Hi there, I'm not sure if anyone has used that site (I haven't) but I'd imagine it'll work like any other job site, only difference is youre applying for an apprenticeship. The process goes something like.. you see the job, you submit your CV, covering letter etc.. That goes to a recruitment agent who filters applications depending on their suitability for the role before passing a list of suitable applicants on to the employer. The employer will then decide who they want to interview.
In the case of an apprenticeship, I don't believe that the establishment offering the training gets a say in who the employer can or can't take on. I guess that they may refuse to provide training to someone but I'd imagine that'd be under extreme circumstances.