To really get deep into the topic, you should watch some documentaries about it. Here is the list of the best documentaries about the Soviet experience in Afghan which you can watch online for free.
How The Afghans Defeated The Soviet Union | Full Hikma History Documentary
Really good all-round documentary about the Soviet-Afghan War. It features all stages of Afghan history in XX century, explains how situation ended up with the invasion and summarizes the aftermath effect. Really good documentary for someone new to the topic, or to anyone who actually wants to get a deeper understanding of social and political changes that Afghanistan had to go through.
Soviet - Afghan War - The USSR's Downfall by Warographics
Really short, yet sharp video on Soviet experience in Afghanistan. Produced in contemporary style, in under half of an hour this video takes you though all major historical milestones of Afghanistan. It does pay most of it's attention on the actual Soviet-Afghan war, going deeply into the main operations of the war.
Afghanistan 1979 The War That Changed the World by Arte France
A professional high budget documentary made in collaboration of couple of countries. Unlike most other films from this list, it does not focus on soldiers and officers struggles, but actually talks about global politics and features high end officials, who participated in the war from different sides. Recommended for anyone who studies history or is involved in international politics.
Documentary Soviet Experience in the Afghan War 1989 - DUCE Film International
An absolutely fantastic documentary filmed during the Soviet withdrawal from Afghanistan. Interviews with officers and soldiers, a lot of actual footage of the everyday life of Soviet soldiers in Afghanistan make this documentary one of the best of its kind. The narration is limited so you will mainly hear what soldiers and officers have to tell. Strongly recommended for any kind of research on the topic. Oh yeah, the canonic Who Will Save The World by Modern Talking moment is actually from this exact movie.
Operation Magistral, Afghanistan (1987/88)
Not a documentary par ce, just a long footage compilation about one of the last big operations launched by the Soviet Army in Afghanistan. It features mainly paratroopers preparing and going on fireforce missions. This piece of documentary is great for research on combat uniform and equipment.