Weight Paint

How can I copy the weight paint of, say, my left arm to my right?

in the vertex groups list, you can select the vertex group, click on the dropdown menu on the right of the vertex groups list and Copy Vertex Group, it will automatically duplicate it, then select the copy and again, dropdown menu > Mirror Vertex Group, rename it with R suffixe

also it looks like there’s a Fast Carve Addon that does it