can wearing covid mask like n95 cause brain damage or heart damage (like enlargement) due to reduced oxygein in the blood?

Of course not. Doctors wear them all day long. Masks are perfectly safe. 

Three million people have died from covid, so put your mask on whenever you are around other people. You will live longer. 

Considering it doesn't meaningfully lower oxygen saturations, no. 

Doctors and Nurses have been wearing them for decades.

Are they all dead ??????????

Yes. Any mask that obstructs fresh oxygen going in your lungs can do that.

Nope. 

Problems breathing while wearing a mask are generally mental not physical.

Unless it is a mask for particulates (dust, ash, etc) and has become blocked and needs to be changed.

Masks should be changed at least every 4 hours or when they become either moist or blocked.

Wearing?  As in covering your mouth and nose?  All of the thousands of people that uou've seen thousands of people with them were alive.  It wouldn't be good for you to swallow one, though.