If I add, for example, a transaction. How can I know which tables are modified so that when I am developing an extensions, I know exactly which tables to update?

Best way is to log database transactions (LN UI: Options - Debug bshell - DBD/SQL Tracing - Database Actions + Open/Close Tables = BDB_DEBUG=0202 + General Options note the Log name and check appropriate General Options), but be aware, not all updates are equal. For instance the update path may vary when different type of items (BOM, Cost etc) or BPs are used. So figuring out of one or several update routes may not be sufficient to know them all.

If I add, for example, a transaction. How can I know which tables are modified so that when I am developing an extensions, I know exactly which tables to update?I may be wrong, but usually I think the cascade of updates that may be triggered by feeding new data (even via a BDE/BOD) may be triggered automatically by the standard DALs (that remains active underneath the extension). So first I would try to play ignorant and see what happens when you trigger your extension...