My iMac is 21.5 inch late 2012, whether it is support extend display? and why there is no " arrangement " tab on the display

Yes Mac’s have supported Extended Desktop Mode for many many years.

The Arrangement option is only offered when you have an external display connected and Mirroring is turned off.

Now it sounds like you are trying to use an iMac as a display for your MacBook.

If so, please carefully, read > https://support.apple.com/en-us/HT204592

Yes that iMac running Catalina 10.15.7 supports Arrangements with Dual Displays.

see > https://eshop.macsales.com/blog/64405-use-external-monitors-with-macs/