Majority do it solo. Just look up 3 fund portfolio. You don't need any sort of financial advisor until you get into fairly high net worths or have trouble planning yourself when you start to pull from investments. Even then you should go to a fee only (charge by the hour) fiduciary.

Most of the time a wealth manager is just siphoning a percent of your money for something you could easily do yourself with just a few hours f research. Their one percent fee may sound small, but that's $10,000 a year if you have $1,000,000 and they take their cut weather your money is up or down. Others take a fee everytime a trade is made. So on theory they are motivated to make trades to get a commission even if it's not good for you (see wolf of wall street for an extreme example of this, though those are brokers not advisors, but same idea).

Vanguard, Schwab, and fidelity seem to be the two biggest brokerage and index fund companies used here. I am partial to vanguard because it is owned itself by the funds.