I am really getting crazy
You should not.
Meanwhile, I am still not understanding what exactly it is that you are trying to achieve.
after mmc dev 1
mmc dev 1 just changes the currently selected SDHCI controller instance for any subsequently entered U-Boot commands in that session. It won’t persist or anything.
I booted up my board from SD card properly
But how? What exactly is it that you did?
then I tried to boot it from emmc, to setting my environment default “env default -a”
it booted from emmc, But now mmc dev 1 is not working, it just saves and switches to dev 1 but after resetting it is not booting from SD card.
As mentioned above
mmc dev 1 by itself really does not do much of anything.
I really did not understand HOW TO BOOT FROM SD CARD and HOW TO BOOT FROM EMMC
Some more information about Linux booting and various options may be found here:
I want to run my operating system from external SD card or EMMC, how to switch between them when I want, please help me TORADEX…
You may also find this new article about Distro Boot useful:
Plase clarify me.