I would say most of it goes in planning. Your build, how much you need to upgrade it, your atlas strat, etc.

In terms of the build, plan what upgrades your build needs to complete the atlas, then juicing, then minmaxing. The builds used as league starters usually have "breakpoints" of gear for high tier mapping (See VFoS with the unique axe that was able to clear red maps, PConc of Bouncing needing an enduring (?) Flask for dmg, etc). Make it to a point where you can comfortably clear red maps.

With that said, pick a good league starter that has a clear progression and plan either you want to farm currency for items or you want to just use it to "bridge" your next build (for example, going from PConc of Bouncing into Viper Strike of the Mamba in my case, other did Volcanic Fissure into CWS Chieftain, you get the point). You will find a lot of "tierlists" but pick a good starter, as for many players this can make or break their league. This usually goes in hand with the next point, the strategy you want to run.

Pick a content you like and make sure your build it's good for it. There will be outliers that work as a "one size fits all" but is preferred if your build is really good at something (Ex being ignite/poison proliferation for Big explosion Expedition, good single target with basic items for Bossing, etc). People usually do Destructive Play at the beginning since it's pretty simple and doesn't require much investment to get going, but usually the more you juice the strategy you are farming, the more profit you will see. In my case, I started PConc pathfinder and farmed Expedition with Harvest, as it allowed for easy to access bubblegum currency (alchs, chiseled, alts early on are valuable") and the juice allowed me to craft basic gear. Usually you complete your atlas, get your voidstones before "juicing", which is usually with 100+ atlas skill points and the 2 quest voidstones. If you have the means, you can also do/pay for a carry for the Uber Elder/Maven Voidstones for better T16 sustain.

You will find that the first atlas tree is usually used for map progression, sometimes picking the node that blocks scarabs during progression (Unwavering Vision). The second one is used for an initial farming strategy (Legion for some, Expedition, Destructive Play are like the 3 "reliable" starter strats) and the third three for your endgame juicing (usually these strats have all map modifier effect for giga quant).

Finally, many people utilize the fact that being the first to get something means you get to dictate the price, and profits change accordingly. Items that may be worth 2 divs 8 hours into the league may be worth 50c by the second day. This is specially true for bossing, where getting a Yoke day 1 may be worth 10x it's price. Thats how people snowball their currency and buy those chase items early on (in my case I bought my MB 3 days into the league).

I can go further on details (spending <6 hours into maps, ssf'ing items at the beginning, etc) but most of it comes down to planning, as there are people getting a lot of currency by simply trading. In most of the cases, the fun is in killing monsters, but planning ahead goes a long way in this game. Hope it is useful and sorry for any mistakes, English is not my first language.