My phone is stuck on 1 percent

my phone is stuck on 1% its kept turnin gon and off every 3 minutes but when i took it out off the charger it stopped turning on and off amd shut down completely as am writing this i put it on the charger and it opened and i got so exited 20seconds later it shut off all my hope was gone off it turning on again its a new phone too buti think i know what the reason is it got cracked idk where (it used to be my dads phone) he went to fix it at this store it cost 1500sr which is alot and i think that the people who supposdely fixed the phone put a bad part in it and i think that part is the battery and the screen because when i go into the settings it tells me "this is unknown part etc"and by unknown part its talking about the battery and the screen now i cant solve hw or log into social media acounts because everything is connected to my phone so if i try to log in to an acount it sounds a notifaction to my phone which is broken