[D:I] Farm it like a real solo arpg

https://www.reddit.com/r/DiabloImmortal/comments/wkmtey/platinum_farming_guide_pc/

Farm Platinum in D:I like paragon xp in D3.
You need multiple accounts, but you can solo farm platinum, un-capped, as many hours you want. Most game features can also be completed solo with this setup.
NetEase may make changes though, because this is obviously not what they intended for this money grabber.

Is platinum server/account-wide or toon-exclusive?

Dunno about Netease but alt farming is very common thing to do for Mobile ARPG F2P players.

Farm it like a real solo arpg, just ignore other player stealing your mobs or best loot for kill shot and just ignore being forced to group up for dungeons and several activities like Helliquary. If you believe really, really strong to be solo, you feel playing solo, even in groups or with player stealing your kills. xD

Make the party yourself & then kick them just before the thing dies so you get the killing blow.

You have not read the reddit post. In this setup, you are mainly farming dungeons in a 4 player team, with all players being your own alt accounts. Three of them are not doing anything, so this is not multi-boxing. You can solo dungeons and heliquary. Outer world farming is optional.

Platinum is the currency you use to buy gems from the market. Gems are what make your character stronger in D:I. Everything is hero-bound in D:I but there is the market loophole which allows sharing of platinum server-wide.