He effectively became his own boss in Season 5, with Gus Fring eliminated. He grew more and more arrogant, but realised he didn't have the operation that Gus had, and thus couldn't hope to reach as many people as Gus did.
As a result, he works with Lydia, an executive for Madrigal, the parent company of Gus' Los Pollos Hermanos. She takes care of distribution for them and we are told on a few occasions that the drug is hugely popular in Eastern Europe.
Evidence of how popular it is can be seen in the gang's theft of a thousand gallons of methylamine (compare that to their theft of a barrel of it in Season 1, with a barrel being roughly 160 litres).
Following this episode, and Todd's murder of the young boy, Mike and Jesse try and end the partnership with Lydia and instead sell to Declan, a competitor based in Phoenix.
When Walt arrives at the meeting, he is able to convince Declan to sell his product, citing its popularity and purity. This works, so Declan is now a distributor of theirs.
Shortly after, Walt learns Mike is on the verge of being arrested and after an angry confrontation kills him. It's after this that Walt goes back to Lydia to establish a deal to distribute his drugs in the Czech Republic.
All of this occurs in Part 1 of the Final Season. We don't see/hear too much about Lydia's operation in Part 2, but it's obvious that the drug has a sizeable presence in Eastern Europe and is being distributed by Declan (until his unfortunate demise!).
Therefore, in answer to your question - Walt earned his $70 million+ through the distribution of Declan to an extent, and far more importantly through the distribution provided by Lydia in Eastern Europe.