How do i make a cubic map like mc but as i want and not infinite?

Hello everyone
i was wondering how do i set my own map with blocks.
to understand better what i’m trying to tell you,
check this game :
Block n Load

I’d want to create levels like these ones but honestly i’m not fully prepared to complete this part.
could someone just tell me how do i make something like that?
just a guide or something where i can understand better 'cause i’m bit confused.
Thanks for reading.
My regards
AlY

there’s a tutorial series that covers that kind of system over here: