Life is very convenient alone
You can eat, shop and travel solo without anyone noticing. Nothing forces you into conversations, so you have to opt in.
Short answer: it's slower than in many countries, but far from impossible. Most people who say it's hard are really describing two things: Japan's daily life doesn't push you into meeting people, and friendships here often take longer to warm up. Here's what's going on, and what helps. If you speak English and you're in Tokyo, the easiest way in is Buy Me a Matcha, the only app built specifically for English speakers to make real friends in Japan: friends-only, one-on-one over a matcha.
You can eat, shop and travel solo without anyone noticing. Nothing forces you into conversations, so you have to opt in.
Colleagues are friendly, but weekend plans with coworkers are less common than many people expect.
Many people in Japan take time before inviting someone into their personal life. The second and third meeting matter more than the first.
It matters less than people think in Tokyo, where many people speak English, but it narrows the pool in smaller cities.
For many people, the first month often feels busy and fine. The quiet usually hits around month three to six, when the paperwork is done and weekends start to feel empty. People who keep meeting someone new every week or two tend to have a real circle by the end of their first year.
Buy Me a Matcha helps you meet new people in Tokyo one at a time over a coffee or matcha. In English, friends only, free on iPhone.

It's often slower than in other countries, mostly because daily life in Japan doesn't push you to meet people and friendships take time to warm up. Going somewhere every week, suggesting specific plans and meeting people one at a time all help a lot.
Usually yes. Tokyo has a large English-speaking community, more events in English and more people who are also new to the city.
Many people feel settled socially within six months to a year, especially if they meet someone new every week or two.
Not in Tokyo, where many students, teachers and professionals speak English. Learning some Japanese helps with Japanese friends and life outside big cities.