I was advised to get everything that I need first otherwise the electrician might either A) over charge me if they have to procure the parts, or B) have to make multiple trips to my house due to not having on hand what he needs to complete the job.
I think you got it back-asswards.
Find a good, reputable electrician. That solves those problems. I had the company I hired for the generator here before for a number of other projects. It’s family owned and I deal with the main guy and his dad (who founded the company). They have a GREAT reputation locally and I totally trust them to do quality work and at a fair price. They aren’t the cheapest, but I know their work is high quality.
The chance the electrician will have everything on hand to complete the job, even if you buy some materials up front, is vanishingly small. He WILL needs a trip to the supply house once he assesses what is needed.
You need a consultation / bill of materials / pricing trip, then an installation trip after he acquires all the materials.
Honestly, the guys suggesting you should have engaged an electrician up front are probably correct. I wouldn’t, but as you probably can figure by now, I’m not the average homeowner on this stuff.
The dealer/store that sold you the generator can tell you exactly what additional items you will need to get it installed...
A reputable & licensed & American English-speaking electrician will handle notifications of the power company and any relevant local governmental organizations about what he is about to install...