I might give you my experience from the other side of a table - how I was introduced to the codebase in my last job. On the first day, I sat down with my lead

I might give you my experience from the other side of a table - how I was introduced to the codebase in my last job. On the first day, I sat down with my lead and for about ~3 hours he was explaining to me the whole codebase. It was overwhelming but the codebase wasn't that big (~18k LOC). I'm not saying that the approach was good/bad. Just sharing my experience. I wonder whether this approach is common.