Be interested in your employees. Ask questions and be curious. Certainly the more experienced employees have accumulated a wealth of experiences over the years and often have very interesting stories to tell (for example about other employers where they have worked) from which you could learn a lot.
This also gives the employee the feeling that they are seen, which is essential when talking about employee happiness. This starts with the job interviews. In all the companies where I worked, where I only met the CEO on the first day that I worked there, those were ultimately those with a poor corporate culture.
One of the things that an employee finds most important is that he or she gets a sense of appreciation. I have had many conversations with disgruntled colleagues who were about to quit and over and over again I heard they missed this feeling.
As an employee you put your whole heart and soul into your work, and you often build impressive things. If no sign of appreciation follows, it can lead to a feeling of dissatisfaction. You go to work with less pleasure and research has shown that happy employees are clearly more productive. A simple "you did well, this helps us tremendously!" can work wonders, provided it is serious.
To enable a good team feeling, it is important that there are regular meetings in which the latest news is discussed. How the company is doing in terms of business. Where new colleagues are introduced to the entire company. Where we say goodbye to colleagues who leave. And where each department can show what they are doing. This way everyone is well informed about what the rest is doing, and people can also do their job better, because they understand how their part fits into the overall picture.
Make sure your door is always open if employees want to come with an issue themselves. Try to actively stimulate this. Most employees will use it sporadically at most, but knowing that it is an option already does a lot. I have experienced at a company where I had just worked for 3 months and that my e-mail (where I told how I liked working at this company so far) was not replied to at all. And it turned out that this was a structural problem. This type of behavior guarantees unhappy employees.
Starting with a new employer can be exciting. It is nice if you are assigned a "buddy" in the early days (someone who has been working there for a long time, but who works in another department) to whom you can ask the less urgent questions and someone who can help you with the new environment.
It is also very rewarding for the buddy themselves that they are chosen for this and that they can help someone.