As background, when dealing with blocks and packages, there are four potential directories where data can be located.

There are two "Concrete5 default code" directories:

(web home)/concrete/blocks -- This is where the default pre-installed blocks (Youtube Player, HTML, et al) are housed. If you have to make a modification to any of the pre-installed blocks, you can find the code here. You can also add new blocks to this directory and everything will work just fine, but this is not recommended.

(web home)/concrete/packages -- This is where the default pre-installed packages are installed. Since there are none, this directory should be empty. You can add new packages to this directory and everything will work just fine, but this is not recommended.

There are two "Concrete5 customs code" directories:

(web home)/blocks -- This is where any marketplace installed blocks will go, as well as where you should put your customized block data. To my knowledge, since all items off the marketplace are packages, and since we all should get into the habit of "packagizing" our blocks, this directory will probably be empty most of the time. I personally use it to test blocks and code snippets before adding them to a package.

(web home)/packages -- This is where any marketplace installed packages will go, as well as where you should put your own customized packages. Anything downloaded from the marketplace will be found here.

One of the design philosophies of packages, especially ones from the marketplace, is that the packages be totally modular, meaning that they can be thrown in and out of the (web home)/packages directory without needing any extra install steps or work from the user (there are exceptions, but this is discouraged). The main driving force behind this is to make life as easy as possible for system administrators and website hosts/masters who have little or no knowledge of coding or complicated setups. So from the philosophy above, when an item is installed from the marketplace, there should be no additions or modifications to any code in the (web home)/concrete directories. Any code from the core Concrete5 that does need to be overridden in the package doesn't actually modify the core code, but has necessary changes to the core code added in the package directory. One of the advantages to using Concrete5 is that Concrete5 knows when running the package to use the libraries found in the package itself first, before using the core code.

As for the database, whenever a block or package is installed, the database is updated according to instructions found in a file titled db.xml, which is located in the home directory of each block within a package. So, for instance, if you have a package called "alpha", containing two blocks named "beta" and "gamma", you should see two db.xml files, in the following places:

(web home)/packages/alpha/blocks/beta/db.xml
(web home)/packages/alpha/blocks/gamma/db.xml

The standard is to have these files create a table in the Concrete5 database for each block, with the naming convention "bt(capitalized block name)". So using the example above, on the installation of "alpha", we would see two new tables in the database, one titled "btBeta" and the other titles "btGamma".

Aside from that, there are several tables that are modified when a block/package is added and subsequently whenever a block is added or removed from a page. Because these tables are handled internally within Concrete5, there is usually never a reason to modify or manipulate these tables manually. I can list exactly which tables these are however if you request.

Hope this helps.