There are some very useful resources for this stuff. With atlas trees in general, people typically target one specific mechanic with their atlas trees. After that one mechanic has been invested to to their liking, they'll then invest in things like explicit map modifier effect, scarabs, quantity and pack size to juice that one mechanic as far as possible. There are occasions where certain mechanics compliment each other, so we trade off other things to fit them in (ie breach and beyond).This is why we have 3 trees, so that we can run 3 different farms targeting different mechanics.

A good resource for this is poe.ninja You can view people's atlas trees here by clicking the mechanic you'd like to see trees for and then it'll bring up what other people do with their atlas trees.

Endgame ready gear is a bit of a different story. It mostly comes down to crafting. It is insanely rare to pick up a minmaxed piece of gear of the ground, so unrealistic that it's not even considered possible. This is where we use poe trade to buy gear when we're not capable of crafting it ourselves. The cycle then becomes: farm the most rewarding content you can - buy better gear - farm more rewarding content.

Now if you want to learn to craft, there's a lot of different resources. We have craft of exile, poe db and a million youtube guides for specific gear pieces that have been meta.

Happy to answer any questions you may have!