How can a block regarded as confirmed in bitcoin?
Let's say if A has 1 btc, and A sent 1 btc to B as transaction T1 and also sent 1 btc to C as transaction T2. It is possible that these 2 transactions are written into 2 different branches.


Yes. That's possible.

But there is still no double-spend because only 1 branch will be the accepted one in the future.

Assume that there are more miners. If miners 3, 4, 5 and 6 start mining on the second branch, it grows faster.
After 2 hours, branch1 has 1200 blocks, but branch2 has maybe 1209 blocks.
The chain/branch with the most block is the relevant chain. So miner 1 does not even want to mine on his branch anymore because he knows that the others will ignore this blocks 1201, 1202, 1203.
So miner 1 also switches to branch2 because otherwise, the only wastes electricity.