How to group multiple conditions in an if statement in fish

You can use begin and end for conditionals as well:

From fish tutorial:

For even more complex conditions, use begin and end to group parts of them.

For a simpler example, you can take a look at this answer from stackoverflow.

For your code, you just have to replace the ( with begin ; and the ) with ; end.

#!/usr/bin/fish

if begin ; false ; and true ; end ; or true
    echo "true"
else
    echo "false"
end

if false; and begin ; true ; or true ; end
    echo "true"
else
    echo "false"
end