meson.build exists in more than just one dir. / place
 

As noob in build system meson please be patient to following question
Why meson.build exists in more than just one place in a project dir., are all the same, if not, what usually is the difference ?
How is on meson build a new directory is created ?

Thanks

See description of subdir() in the Meson Reference Manual.

BTW, have you never before encountered a project with multiple makefiles? Then 5.7 Recursive Use of make and 3.3 Including Other Makefiles in the GNU make Manual may surprise you.