Welcome to Android Central forums. We're going to have to back up a little to get more information before trying to jump into this so let's start with a few questions.
What phone do you have?
What model and year is your car, if this is a factory head unit? If it isn't a factory unit, what brand and model is your head unit?
Is the dashcam factory installed, or aftermarket? If aftermarket, what make and model is it?
When you say the dash cam is connected to the head unit, how is it connected? Cable/WiFi/Bluetooth?
When you say you want to connect the dash cam to your phone, do you mean for playback, live viewing, or something else?
What is your goal for connecting all of these at the same time?
You mentioned an app, does the cam have an app specific to it or are you asking if there is an app to connect any or all of these?
What makes you believe that connecting them all together at once is even possible? Is this a stated feature of the cam or head unit?
Have you checked YouTube?
It is nearly impossible for us to really offer you any informed advice without knowing more about your goals, the equipment and how they are connected to one another. I asked if you had looked at YouTube because if it can be done there is more than likely someone out there telling you how they did it or trying to help others do it. Of course, your odds go up the more popular the devices involved are. You're not very likely to find a video for a WooFang si1843 Chinese model dash cam ordered from Temu but you may find something similar that will explain the ins and outs to get it working. We can help with this but we need that information.
Tell us a bit more and we'll offer some advice.