This is an individual problem and an individual's responsibility to solve, imo (Although I do think it's interesting to consider whether a project, company, or initiative could help make this easier to solve for millions.)

Regardless, there are four steps worth taking as an individual: (1) go out, (2) make friends, (3) turn friends into community, and (4) maintain community.

If you're feeling lonely, you're probably failing at one step along this chain.

1. Going out. I don't have a lot of tips here. Except to go to things that actually facilitate interacting with strangers. Don't just go to a bar or go work from a cafe. Go to a meet and greet, an event for strangers to mingle, etc. Or, if you're having trouble motivating yourself to go out, then that's something inside yourself to work on. I find that a shakeup to your life routine (e.g. moving cities, going on a vacation) can provide a good window to change your habits, where you'll start doing things you don't normally do in your home city.

2. Making friends. This one is simple but hard for some. Basically: be personable, smile, engage in conversation, ask questions, be interested, avoid being threatening or clingy, dress and stylish normal-ish unless you really don't want to, etc. Then talk to people at these events, and if seems like you'd like hanging with them and have things in common, ask to exchange numbers.

3. Turn friends into community. IMO this is where you go from the basics into the advanced, and where the most benefits lie. However, most people stop after #2, even though this step is easier than steps #1 or #2, and is extremely rewarding. Community is an in-person social network. The number of connections between people in a community determines the strength and stickiness of that community. Thus it's very important that you introduce your friends to other friends. For example, instead of going on a coffee date with a friend once every month or two, invite 2 or 3 friends to dinner. This has numerous benefits. All of your friends will meet each other, and suddenly they'll know who you're talking about when you mention other people. Also, conversation is easier when there are more people. Also, you'll find events and hangs happen more often, because (a) more people are able to initiate them, and (b) there's more reason to go. People are more motivated to go and less motivated to cancel when there's an event that allows them to see multiple friends at once.

4. Maintain community. People move away. People have silly fights and disagreements and stop talking to each other. People get into relationships and disappear. People get sick, or old, or antisocial, and disappear. Shit happens. So you have to keep doing steps #2 and #3, at least occasionally, forever. You don't necessarily need to do step #1 as much, since the people in your community will naturally bring friends and whatnot to your events. But you still need to get to know these people, exchange numbers, and invite them to future events.