Long answer short: they own businesses and real estate. Now, let’s look more closely at wealthy households in the US, starting with the top 1% (in net wealth):
In 2016, the dollar cut-off for the top 1% was $10,374,030.
Close to three-fourths of the top 1% own businesses.
Close to three-fourths own real estate other than their primary residence.
Almost half of the wealth of the top 1% is in unincorporated business equity and real estate.
Business equity includes cash, accounts receivables, inventories, vendor receivables (e.g., rebates), property, plants, equipment, and marketable securities such as government bonds.
A good number of wealthy business owners are financial, legal, healthcare, and real estate professionals.
Among the most profitable businesses are accounting firms, financial services, legal services, real estate agents and brokers, outpatient care centers, offices of physicians, dentists, and other healthcare practitioners, and medical and diagnostic laboratories.
About 27% of the wealth of the top 1% is in corporate stock, financial securities, mutual funds, and personal trusts.
For comparison, let’s look at US households in the next 19% of net worth:
In 2016, the dollar cut-off for the top 20% was $499,263.
About a quarter own businesses.
Close to 44% own real estate other than their primary residence.
Their primary residence is their biggest asset (28% of net wealth), followed by business equity and other real estate (24.2%), pension accounts (21.7%), and corporate stock, financial securities, mutual funds, and personal trusts (16.3%).
The Census Bureau just came out with some fascinating data on US households broken down by net worth - unfortunately it’s in spreadsheet form, so quite a slog to comb through. One thing I did gleam from the data is that over half of the households in the top 20% of net worth are not in the top 20% of income. That makes sense given that so much of their wealth is based on the value of their homes and businesses. Think of farmers classified as middle-class by their income and wealthy by the value of their land and equipment.
References:
Wolff, E. N. (2016). "Household Wealth Trends in the United States, 1962 to 2013: What Happened over the Great Recession?" RSF 2(6): 24-43. https://www.rsfjournal.org/doi/pdf/10.7758/RSF.2016.2.6.02
US Census Bureau, Survey of Income and Program Participation, 2014 Panel, Wave; https://www.census.gov/data/tables/2014/demo/wealth/wealth-asset-ownership.html