1. The equipment the general public can buy on their own is about a 3 on a scale from 1 to 10 as far as quality, real efficiency, operation and longevity.
2. This work is MUCH more difficult, complex and tiring than most can imagine. Couple that with ever-changing rules, regulations, etc., and you can get in trouble quickly.
3. Proper sizing, routing, workmanship, etc., comes from experience, knowledge and years of paying attention, asking questions and retaining knowledge.
4. Every brand is different with regards to controls, operation, etc. Most of it "all works the same" to achieve desired results (or get close enough).
5. Most people don't even know where to start with regards to purchasing equipment, supplies, controls, etc. But I am always floored at how many people come up with capacitors for condensing units on a Sunday afternoon, so there's that.
Many high quality efficient systems can be bought from various vendors online. I bought my 3 ton Mitsubishi hyper heat system (4 zone with a branch box), all premium mueller line sets with 1/2” insulation, and disconnect box/surge protector all from the same vendor. I got all my wiring from other online sources (south wire shielded in conduit with protective coating). I also purchased my refrigerant through an online vendor.
If you know how to do manual J calculations or have previous experience with different systems/btu ratings from past use, sizing is not hard to figure out (my home is also fully air sealed and insulated).
Being a tradesman already (not HVAC, though) a lot of it is common sense after doing lots of research on code, proper line routing etc. it is far from rocket science. I have lots of AC experience in the heavy truck/automotive industry and have my refrigerant certification. If you under stand how systems work again it is not hard at all.