As I was looking at the panel, there appears to be like 400 amps of breakers in there.
If I connected something to each outlet and ran all appliances would the panel melt, overload, explode, start a fire etc...?
It would be very difficult to "load" every individual circuit in breaker panel to near it maximum unless you had a couple of electric heaters or a couple of window air conditioners on each circuit. Highly unlikely that will ever happen.
(I have done it, but that was because the house was wired in the 1950s and electrical codes were very lax back then. Nothing "bad" happened. The breaker tripped as it was designed to do. A house built to today's standard would have about twice as many breakers.)
The one room in you house that MIGHT have several high current items running at the same time, would be your kitchen. Coffee pot, toaster, electric frying pan, etc. Electrical code for many years now have required that kitchens have 2 separate circuit, one on the left side of the sik and one on the right. These 2 circuits are not to be used for any purpose other than counter top appliances. So your stove, microwave, refrigerator, dish washer, garbage disposer and lights are on different circuits.
I have a 200 amp service to the house. I had an electrician replace a 25 amp breaker with a 30 amp for my water heater.
This could be potentially dangerous ! The wire between the breaker and the hot water heater was
(likely) sized to handle 25 amps continuously. At over 25 amps continuously, the wire could over heat, melting the insulation and then REALLY BAD THING could happen ! Which is why ...
BTW- why did the electrician replace a 25a with a 30a breaker?