Cognitive behaviour therapy is one option to address loneliness. More information can be found here. "Therapist-Guided Internet-Based Treatments for Loneliness: A Randomized Controlled Three-Arm Trial Comparing Cognitive Behavioral Therapy and Interpersonal Psychotherapy" (https://www.karger.com/Article/FullText/516989).

Other options are mindfulness. "Mindfulness training reduces loneliness and increases social contact in a randomized controlled trial" (https://www.pnas.org/content/116/9/3488).

And community navigation interventions. "The Community Navigator Study: Results from a feasibility randomised controlled trial of a programme to reduce loneliness for people with complex anxiety or depression" (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7259554/).

The strategies will depend on the different needs of the clients. For example, an elderly bereaved person who is lonely will need a different strategy from someone who has social anxiety, who again will be different from someone with autism. The principles will be the same, but the key strategies may differ.

This is a difficult problem. I wonder if this book may be helpful? "A Sense of Belonging: How to find your place in a fractured world" by Dr Holan Liang (https://www.amazon.co.uk/Manifesto-Belonging-Place-Fractured-World/dp/1780724683).

This speaks to the first point. For some people who have social skills deficits, social skills training may be helpful. For others, their primary difficulty will be more about their perception of the discrepancy between their perceived and desired social relationships.

Again, there is a lot of individual variation. For some, social anxiety is central to social isolation and loneliness, but for others, it is less relevant.

This is hard to answer. Some people don’t have any friendships since college and school, and their closest friends are those they make in adulthood (e.g., when becoming a parent or in the workplace).

This sounds like a complex situation. New parents are also exhausted and going through lots of other changes. Accessing support for new parents and helping people manage difficult emotions that become overwhelming when alone are important issues that may require interventions broader than just loneliness.

It depends on what you mean by 'connected.' CBT for loneliness can help people address personal meanings, e.g., perceived lack of connectedness. Emotion regulation training may help give more insight into emotions and acceptance of them. These may be good resources.

"Emotion regulation in psychotherapy: A practitioner's guide" (https://www.amazon.co.uk/Emotion-Regulation-Psychotherapy-Practitioners-Guide/dp/1609184831).

Mindful self-compassion (MSC). In the "Handbook of mindfulness-based programmes" (see attached PDF).

It depends on whether it is a thought or a fact. If it is a fact, then increasing social contacts will be a priority. If it's a thought that is not based in reality, then understanding the meaning of that thought to the person would be a central part of helping to change it.