am curious how do said tapes even withstand erosion?

Tapes are honestly robust. Not many people know that so they tend to give up on their tapes if they get flooded, dirty, or the ribbon gets cut. Plenty of VHS got discarded during Ondoy when it could still be remedied. Dust, molds, and sticky shed - they can be cleaned. Huge pain in the ass, though! Here's me cleaning one that's super dirty.

and do you mind explaining the restoration process I imagine it ain’t cheap?

When I first started, I just bought a cheap VCR and a digital dongle that's around 2k all in all. The results are okay, but you run into problems when the tape is in bad shape. I have since learned a lot and upgraded to better equipment.

It's expensive (due to rarity), but luckily for me, VHS restoration isn't a thing here so you can buy stuff much cheaper than those in the US. I was able to get an S-VHS player here for 9k when on eBay it runs for 84k++

Digitizing is real time. I copy it losslessly, upscale it a bit to 720p, then compress to mp4. I can't afford a frame TBC (device used to clean VCR signals that was pricey then and is more pricey and rare now) so sometimes there's not much I can do with super old and poorly-recorded tapes even if I have 80% of the highly recommended capture setup.

also gow do you even come accross these tapes

Most of it is from the little I was able to save from my own recordings. Used to have like 60 tapes but only set aside 15 when we remodeled the house a long time ago. The rest, I buy off Carousell or FB for dirt cheap because the seller just wants to get rid of it. Some are donations.

I regret getting in the hobby late in the game. Those who own recordings likely disposed of them many years ago. You also have to contend with cartels who buy tapes just so they're the only ones who have copies of any lost media.