How to map a MacBook keyboard to write different characters?

if you go to

system preference>keyboarder>input source

then click on >> option key (alt)

you are going to see all even in Japanese

I think the characters you need are already there if you use the US or ABC layouts. Using what is printed on the keys of a Japanese keyboard,

Tilde can be had by typing Shift ^ . ~

Backtick can be had by typing Shift @. `

Both of these are actually printed on the keys.

If there are others, let us know. If you really need to use the British or Australian layouts instead, this won't work and you should probably make a custom layout based on one of them with Ukelele, which has a template for the JIS Japanese physical arrangement.