Is there a word or expression for someone who knows various things, a little bit of everything, but is not specialised in anything?

They're a jack of all trades. Some might add and master of none.

"Jack of all trades, master of none" is a figure of speech used in reference to a person who has dabbled in many skills, rather than gaining expertise by focusing on one.

Such a Jack of all trades may be a master of integration, as such an individual knows enough from many learned trades and skills to be able to bring the individual's disciplines together in a practical manner. This person is a generalist rather than a specialist.

- wikipedia

If you are looking for a term that describes a person as not being a specialist, but doesn't necessarily place any limits on their level of skill, then you can call them a generalist.

Generalist
Noun

One whose skills, interests, or habits are varied or unspecialized

^{Merriam-Webster}

Emily Wapnick calls it a Multipotentialite in her TED talk.

Also known as a polymath, Renaissance person, Jack of all trades, person of many talents, etc.

Excerpt from her talk:

A multipotentialite is someone with many interests and creative pursuits. It's a mouthful to say. It might help if you break it up into three parts: multi, potential, and ite. You can also use one of the other terms that connote the same idea, such as polymath, the Renaissance person. Actually, during the Renaissance period, it was considered the ideal to be well-versed in multiple disciplines. Barbara Sher refers to us as "scanners." Use whichever term you like, or invent your own. I have to say I find it sort of fitting that as a community, we cannot agree on a single identity.

https://www.ted.com/talks/emilie_wapnick_why_some_of_us_don_t_have_one_true_calling/transcript

jack of all trades, master of none.