Question about forming an SDP body

[CKOTu]

Greetings.

I cant figure out where sdp body is formed. For example, outgoing SIP call, INVITE. Traced it to desc = await _connection!.createOffer(constraints); in rtc_session.dart. Giving it constaints like {mandatory: {OfferToReceiveAudio: true, OfferToReceiveVideo: false}, optional: []}, it in desc.sdp retuns full sdp body like:

v=0
o=- 3563908965786039858 2 IN IP4 127.0.0.1
s=-
t=0 0
a=group:BUNDLE 0
a=extmap-allow-mixed
a=msid-semantic: WMS cb465a6b-47e5-422e-8287-3575dffb0874
m=audio 9 UDP/TLS/RTP/SAVPF 111 63 9 102 0 8 13 110 126
c=IN IP4 0.0.0.0
a=rtcp:9 IN IP4 0.0.0.0
a=ice-ufrag:B5w/
a=ice-pwd:PPVSgLFFFcujc/hJwr2bBrTl
a=ice-options:trickle renomination
a=fingerprint:sha-256 9E:CC:42:DC:F7:90:B7:70:C5:24:C7:4C:B0:CB:3F:30:C1:A8:FE:29:5A:B0:DD:93:EE:C0:9A:3B:13:80:8B:2C
a=setup:actpass
a=mid:0
a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=extmap:2 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time
a=extmap:3 http://www.ietf.org/id/draft-holmer-rmcat-transport-wide-cc-extensions-01
a=extmap:4 urn:ietf:params:rtp-hdrext:sdes:mid
a=sendrecv
a=msid:cb465a6b-47e5-422e-8287-3575dffb0874 e4a76f3d-ea88-4c09-b79a-612101cc16a6
a=rtcp-mux
a=rtpmap:111 opus/48000/2
a=rtcp-fb:111 transport-cc
a=fmtp:111 minptime=10

By what package and where it is formed?

Thanks for help.