Boot loop on my new iphone

I got a new phone like a week ago from jbhifi brand new nothing wrong with it i was expecting better quality and results since i mostly get my phones from face book and they have been better then the one i got now. Anyways i tried setting it up as usual turn it on get the charger all that stuff but then it turns off randomly and I’m confused so i power it back on and then it turned off again so i power it back on Untill it kept on doing that over and over and it took so long to even set it up. It took me a while to figure it out but now its taking longer to shut down but it still does it say i want to watch TikTok or do something else and I’m watching for lets say 5-10 mins it shuts down and the apple logo appears and it keeps doing that over and over and it doesn’t stop i tried rebooting it, restarting it, erasing everything, force restart everything hasn’t worked I’m severely disappointed as it is a new phone and shouldn’t be acting up like this. Apple please help me out what should i do with it or anyone please respond and help me its brand new and was very expensive.