@kingfranz said in cross compiling for ubuntu 21.10 on raspberry pi 4:
/usr/lib, /usr/include and /opt
Yes, you do not have to copy whole root file system.
"I haven't found any info of what is needed" - what is needed heavilly depends on what parts of Qt you want to build. What you can do is: run configure with your current sysroot, see which tests fail and for those you need check why they fail (check configure.log file) and install missing dev packages.