This is not the easiest setting.
I became American as a previously British citizen. I had been employed in the UK, by a company in California who wanted to relocate me to the US and did (I was an early H1-B).
I then moved to another company after 18 months. Both my first and second companies were applying for my green card and renewing my paperwork as needed for me.
Later I stopped working for a US company but still had a VISA and married a US citizen. I then handled my green card application, and citizenship, myself without lawyers.
As a highly qualified individual who had already been screened multiple times to get H1-Bs I knew I would pass further screening. I also knew I had no criminal records or adverse history globally.
In short I got my green card and my citizenship without any further professional legal help. I paid nothing but my own costs.
It took a couple of years but it is possible.
The process is torturous and repetitive. You have to resubmit the same information multiple times, and some of the requirements are extremely expensive.
To whit, I was required to produce a “Police letter” from every country I had lived in, signed by the local police, attesting to the fact I hadn’t broken any laws overseas.
I had lived in 4 continents at that point. Thus I had to arrange to send my ID to multiple countries and to pay, in some cases, for letters to be written, delivered as originals on paper and then (hilariously) pay to have them translated for a US government who only wants to work in English and apparently trusts whatever translation you send (this was pre LLM).
So though I could do all this, in one case paying an ex colleague to manage the police in Eastern Europe on my behalf, for many others this would be impossible and require lawyers and the huge markups they would charge for these services. I would guess them hiring another team of lawyers in another country with each stage doubling the costs of the ones beneath them meaning a single police letter ends up costing many thousands.
The system is thus absolutely limited to those with connections, deep pockets or sponsorship.
Also for those who think this is good insurance, I also know Central Europeans who bribed their local authorities to facilitate their green cards, covering adverse information and putting them at the tops of lists. Ie for $50k or so they got essentially instant residency status.
Also the need for people to leave the US before re entering when processing paperwork (so that if rejected you have already self deported) means you need to be able to stop working, or work remotely, and to be able to fund living in your old home country for an indefinite period.
I moved in with my parents but had they not been an option I would have had to rent a place in London - a vast expense - just to comply.
The system is incredibly broken.