with insight that none of this is at all useful to the OP....
Surgery is an inflammatory process and causes activation of the clotting cascade by a mixture of mechanisms. Makes sense when you think about it - surgery is an injury and the natural evolutionary response to that is to stop the bleeding...our bodies can't tell though whether the injury is a controlled process at the hands of a surgeon or because a bear just ripped a leg off. Some procedures/types of surgery are higher risk than others (pelvic, lower limb joint replacement, cardiac/vascular surgeries being examples of higher risk categories for elective surgery). The duration of procedure can also affect clotting risk and that will be partly to do with intraoperative immobility, but there has to be more to it than just immobility (otherwise a whole bunch of people who are normally immobile for various reasons would be needing thromboprophylaxis and that isn't the case). Getting up and mobile as soon as possible does reduce the risk of developing a clot, but doesn't take away the risk entirely and mobility is just one of many modifiable factors (smoking and being overweight being two of the other commoner modifiable risks). Surgery and risk of DVT – GPnotebook (hopefully this link works - it's not to anything terribly high-brow, but it outlines some of the basics, with links to other sources).
Every single patient having a surgical procedure will have a venous thromboembolism risk form filled out and based on that risk reduction measures will be taken. For low risk day case surgery on someone with no particular risk factors (a huge proportion of the elective surgical population) that might just include TED stockings/foot pumps while they're in theatre, staying well hydrated and getting back to normal pretty quickly. For other procedures it may include adding in pharmacological treatments (heparins/other anticoagulants) for varying lengths of time and that may be the case even with day-case procedures for which prolonged immobilisation isn't anticipated. The risk of surgical site bleeding due to pharmacological methods has to be factored in on top of the clotting risk and that may affect whether someone gets drugs post-op or not.
If you're really bored, you can google the Caprini score and fiddle about with some numbers for hypothetical scenarios - you'll see that your risk of post-op dvt will change quite dramatically depending on the type of surgery and if you add in/subtract different other risk factors.
Historically it used to be thought that the type of anaesthetic had a big influence in the risk of post op thromboembolism, but that is less clear (partly because GAs are better than they used to be). Intuitively it makes sense that neuraxial/regional anaesthetic should reduce the risk. There is often benefit in avoiding GA if the surgery allows (things like less nausea & vomiting, less cognitive dysfunction, better pain control will all allow for earlier mobilisation).