There are several NGO efforts in Ukraine to identify troops killed in war by name, gleaned from public announcements, mostly internet. These offer estimates of between 58,000 and 65,000 dead. A similar effort by dissident Russians working with the BBC has found the names of 82,000 Russian soldiers killed in war. BBC, 43,000 troops killed in war with Russia, Zelensky says.
The other sources are: Ukrainian losses – WarTears Ukrainian losses – UA Losses Russian losses – Mediazona