Update for Russian military operations in Ukraine for May 21, 2022:

1. Ukrainian militants in the Azovstal steel works have unconditionally surrendered numbering between hundreds (current Red Cross count) to thousands according to Russian sources;
2. Mariupol has been secured by Russia since mid to late-April - Ukraine finally admits however that its “combat operations” have fully ended;
3. Russia continues to make progress in the Donbas region - Ukrainian forces are being encircled in Severodonestsk and Lysychansk - the salient around Papasnya is expanding both west and north as part of this major encirclement;
4. Pentagon admits Russia still poses the majority of its combat power - possess a “numerical advantage” over Ukraine - and is making “slow, uneven” progress, but progress nonetheless;
5. Kharkov “offensive” has either stalled or is in the process of being rolled back;
6. US M777 howitzers have been found, tracked, targeted, and destroyed by Russian forces;
7. Additional attempts to send heavy weapons will face similar technical, tactical, strategic, and manpower challenges;

References

US Department of Defense - Pentagon Press Secretary John F. Kirby Holds a Press Briefing - May 20, 2022:
https://www.defense.gov/News/Transcripts/Transcript/Article/3039550/pentagon-press-secretary-john-f-kirby-holds-a-press-briefing/
US Department of Defense - Senior Defense Official Holds a Background Briefing May 19, 2022:
https://www.defense.gov/News/Transcripts/Transcript/Article/3038153/senior-defense-official-holds-a-background-briefing/
TASS - Leader of Azov battalion transferred from Azovstal plant in armored vehicle - top brass:
https://tass.com/defense/1454085
SouthFront - IN VIDEO: RUSSIAN DRONE, ARTILLERY DESTROY UKRAINE’S NEW M777 HOWITZERS:
https://southfront.org/in-video-russian-drone-artillery-destroy-ukraines-new-m777-howitzers/
Institute for the Study of War - RUSSIAN OFFENSIVE CAMPAIGN ASSESSMENT, MAY 20:
https://understandingwar.org/backgrounder/russian-offensive-campaign-assessment-may-20

Mirrored - The New Atlas