Making friends as an adult takes time everywhere. In the US, people are often friendly at first and become friends slowly, by seeing each other again and again in the same place. The key is to go to the same thing repeatedly.
Meetup and Eventbrite: search for hiking, language exchange, board games, photography, running, book clubs, tech events. Many are free.
Sports leagues: city recreation departments and adult leagues for soccer, volleyball, basketball, cricket, pickleball. Search your city name and "parks and recreation".
Classes: cooking, art, dance, yoga, community college classes. The library has free classes and groups (see the library guide).
Volunteering: food banks, animal shelters, schools, parks (see the volunteering guide).
Places of worship: most have social groups, women's groups, youth groups and shared meals, and welcome newcomers.
Diaspora groups: associations, student groups, cultural centers and WhatsApp or Facebook groups for people from your country. Great for advice and feeling at home.
Parents: school events, the PTA, playgrounds, children's library story time.
Neighbors: say hello, join building events, use Nextdoor or local Facebook groups.
Work and school: say yes to lunch, after-work events and clubs.
How to move from "nice person" to friend
Go to the same group at least three or four times. People start to recognize you.
Ask a person you liked for their number: "Do you want to swap numbers? I'd love to do this again."
Suggest one concrete plan: "I'm going to the farmers market Saturday morning. Want to come?"
Follow up. Americans are busy and plan ahead, often a week or two. A "no" with a reason usually means "ask again later", not "never".
Balance two kinds of friends
Friends from home culture give comfort and language. American and international friends help you learn how things work and improve your English. Most newcomers do best with both.
Waiting for people to invite you. Invite first; it is normal here.
Trying one event and deciding "Americans are cold".
Thinking you need perfect English. Most people care more about your interest than your grammar.