Sorry up front for the ramble, but it's just fascinating to me how many mistakes they've made, and how arrogant they were (of course, the previous 30 years saw them easily handling most of the military situations that arose, and were never this big of an operation against a well-trained/supplied enemy).
Pre-war they had a ground force of 350,000 or so active duty. Breaking that out into logistics, support (food prep, medical, etc.) as well as non-frontline combat arms (artillery, air support, etc.), and all of the sudden you start dropping into a range of maybe 50,000 who are actually dudes with guns who can take and hold territory. That's not a lot when invading a country of 40 million people. And the 2022 invasion force probably had at most 50,000 dudes with guns who can take and hold territory, because it was not the entirety of their ground forces (and it probably leaned heavily on infantry, but still needed the support troops to actually move lots of troops over the ground). Obviously they've boosted those numbers, either pulling trained personnel from other jobs (which hurts the military longterm), or with mobilization, but even if they doubled or tripled the number of dudes with guns who can take and hold territory, it's still not a lot, because they still have to have a lot of guys in the rear doing REMF stuff to keep the guys in the front going.
For reference, we went into Iraq in 2003, a smaller country with a mostly gutted military that hadn't had any new stuff since the 1980s, and we did it with air superiority, with an extremely mobile force of 250,000+, and we still struggled, even after removing the Iraqi leadership and installing new leadership.
If you look at Zeihan's videos about demographics, and he puts a lot into them, Russia is on a downward trend - last year they produced fewer 18 years than 2021, this year they will produce fewer 18 year olds than 2022, etc. so they've got an ever-shrinking pool to draw their normal yearly conscriptions from (all Russian men are supposed to do a year in the military). Conscriptions, not the mobilizations where they call up men outside of their normal military service.
Putin allowed around 700,000+ military-aged men to flee Russia. On the surface, Putin got rid of 700,000 men who might have resisted mobilization and/or protested the war. 700,000 men out of 140 million is not a lot, but 700,000 men out of the pool of 14 million 20-34 year-olds gets to be interesting, especially when those 14 million men are also (in theory) the prime drivers behind the economy, and 700,000 of those who might have been making more money than most just bailed.
Russia also has a problem with training and leadership - their training battalions were gutted to boost manpower on the frontlines, and they rely heavily on men who re-enlisted for another year or two as their NCO corps, and now they've lost a helluva lot of experienced men they could have otherwise relied on to both train incoming mobilized troops, but also provide the backbone of small-unit operations. But it gets worse - their officers do much of what we think of American NCOs as doing, and they've lost 1,740+ officers (confirmed by official Russian government accounts, social media, actual burials, etc.).
The documented losses below are staggering, it's the equivalent of:
800 platoons losing their leaders
330 companies/batteries/troops losing their leaders
120+ battalions/squadrons losing their leaders (in their military they have majors sometimes filling in the Lt. Col. slot)
50+ regiments losing their leaders
Multiple division-level leaders
But it gets murky, because maybe it wasn't 800 platoons losing their leaders, because those senior lieutenants may have been filling in as company commanders, which means maybe more than 330+ companies lost their leaders, etc. Still, they lost 1,740+ officers who not only do officer shit in Russia, but also NCO shit.