Below is the database file which Nmap uses as a reference to packet info specific to OS. It is at /usr/share/nmap/nmap-os-db in the case of Ubuntu 20.04.4 LTS.
# Linux 2.6.30-ARCH #1 SMP PREEMPT Wed Sep 9 12:37:32 UTC 2009 i686 AMD Athlon(tm) XP 2000+ AuthenticAMD GNU/Linux
Fingerprint Linux 2.6.30
Class Linux | Linux | 2.6.X | general purpose
CPE cpe:/o:linux:linux_kernel:2.6.30
SEQ(SP=C1-CB%GCD=1-6%ISR=C5-CF%TI=Z%II=I%TS=U)
OPS(O1=M5B4NNSNW6%O2=M5B4NNSNW6%O3=M5B4NW6%O4=M5B4NNSNW6%O5=M5B4NNSNW6%O6=M5B4NNS)
WIN(W1=16D0%W2=16D0%W3=16D0%W4=16D0%W5=16D0%W6=16D0)
ECN(R=Y%DF=Y%T=3B-45%TG=40%W=16D0%O=M5B4NNSNW6%CC=N%Q=)
T1(R=Y%DF=Y%T=3B-45%TG=40%S=O%A=S+%F=AS%RD=0%Q=)
T2(R=N)
T3(R=Y%DF=Y%T=3B-45%TG=40%W=16D0%S=O%A=S+%F=AS%O=M5B4NNSNW6%RD=0%Q=)
T4(R=N)
T5(R=Y%DF=Y%T=3B-45%TG=40%W=0%S=Z%A=S+%F=AR%O=%RD=0%Q=)
T6(R=N)
T7(R=Y%DF=Y%T=3B-45%TG=40%W=0%S=Z%A=S+%F=AR%O=%RD=0%Q=)
U1(DF=N%T=3B-45%TG=40%IPL=164%UN=0%RIPL=G%RID=G%RIPCK=G%RUCK=G%RUD=G)
IE(DFI=N%T=3B-45%TG=40%CD=S)
The Nmap source code can be found here.