Not sure I'd class it as a science, but here's my take... though I'm no expert and certainly agree this stuff could be better explained!
> Macroeconomics. Central banks are "creating" a trillion here, a trillion there, like nobody's business. But what are the consequences?
Nobody quite knows; it's still hotly contested between left wing lovers of Keynes and right wing believers in austerity.
> What is the thought process that central bankers have gone through to make these decisions?
Probably largely a political one. Central banks may be trying to fulfill a remit set by law (e.g. bank of england: keep inflation below x%) and are trying to deliver on that. (why? too much or too little inflation both cause problems, I guess we somehow reached consensus on a "sane" amount that keeps pace with genuine growth of wealth within the economy)
> Also why, exactly, are they buying the exact assets that they are buying (govt. debt, high-yield bonds, etc..) and why not others (e.g. stocks or put money into startups)?
I think this is about distributed decision making. The central bank does not have the expertise to decide which stocks or startups represent the best investments. The examples here involve lending money to government, presumably the idea being the latter is better placed to decide what to do with the money. Another example is buying assets from other banks, which are again better placed to decide which businesses/homeowners/etc represent a more sound investment as they do it on a daily basis (from a profit/loss point of view ... of course we debate whether or not that's the case on a societal level).
> What would happen if a country defaults on a loan given by a central bank?
Internally it would depend on laws/balance of political power within the country. Between countries, depending on the currency the country could do crazy stuff like print excessive amounts of money to repay the loan (Germany did this in the 1930s leading to hyperinflation) or they could just as you say, default. The country's credit rating would then be downgraded, making it harder for them to raise credit in future.
> Would the central bank then go ahead and seize and liquidate assets of the country under a bankruptcy procedure to pay off the debt (like it would be standard procedure for individuals and companies)?
Not the bank, but the country making the loan, may first negotiate some debt relief with strings attached e.g. preferential trade agreements. Beyond that, I have no idea what precedent exists.
