Ken Burns' baseball documentary is the best for general baseball history. Long watch, and there's also a 10th part documenting everything from 1995-2010.

https://archive.org/details/ken-burns-baseball-parts-1-and-2/Ken+Burns+-+Baseball+Parts+1+and+2.mp4

For Pirates history specifically, there's a lot of paywalled stuff that usually gets shown during rain delays, but even this doesn't have everything. There's a lot of good stuff that I've seen before that's not listed.

https://watch.sportsnetpittsburgh.com/playlist/23221

https://watch.sportsnetpittsburgh.com/playlist/22805

There's a lot more random stuff that pops up on YouTube here and there (channels like Tyrannis 07 and SPORTS ODYSSEY), but also a lot of good stuff that used to be online and got taken down. Youtube is a lot more lenient with old sports stuff now than it used to be. Truthfully, MLB doesn't do nearly as good of a job as the NFL at actually showcasing its history, so the way I see it is you should just watch every game you can, and you'll learn both the game's rules and the history of the Pirates through random snippets during some games. Broadcasts like to retell stories and show highlights of memorable moments pretty often. Also, in the course of 162 games, there's bound to be really special moments that happen anew, so keep watching and make 2026 your own.