Right now, manufacturers often put videos up on YouTube of how to install parts they sell. Like with Ikea furniture, I find these way more helpful than a paper manual. Same for videos about how to disassemble, clean, and reassemble guns (which often link to whatever cleaning products are used) [edit: BTW assembling something that already is a gun still falls under the ATF's definition of manufacturing a gun, so if Youtube uses the same definition these videos are banned even if they don't refer to specific products]

As for the 100 year gun-- let's take the M1 Garand (in WW2 movies, it's the one you hear go "ping". It's only ~85 years old, but bear with me). Over the last decade, the US government has sold off several hundred thousand of these rifles. As far as older guns go, they're fairly common. They shoot .30-06 ammunition, which can be found in most gun stores. However, 85 years ago metal was lower quality. If you use modern commercially available ammo in a Garand it will break [1]. There's a few options though. You can buy surplus ammo and hope that it was stored properly and that 40 years ago the quality control guy at the factory was having a good day[2]. Many people opt to recycle an initial batch of brass and manufacture their ammo. If done correctly this is safe, but there's a bit of a learning curve. Unfortunately the new policy bans videos that detail how to safely manufacture ammo.

[0] channels like ForgottenWeapons often describe how antique firearms work, which might run afoul of the new rules

[1] you can mitigate this with an accessory that reduces the pressure from the gasses. There's some good videos about it on youtube

[2] for more obscure weapons, I don't think this is an option