Usually a 401k will have two options: managed and self-directed. With managed, usually you pay some amount and the custodian selects investments. With self-directed, you pick the funds you invest in.
Usually there are passively managed funds in a 401k. Typically there’s a least an S&P 500 fund, a bond index fund, and often an international index fund. That’s all you need, and nobody is going to sell shares in those funds without your say-so. My 401k allows me to select a target ratio between each fund, so I can have whatever split I want, and I can choose to have it auto-rebalance (I think it costs something) or just contribute new money according to that ratio (free, that’s what I do).
Every 401k is a little different, but 401k custodians have certain fiduciary responsibilities, so they have to provide a reasonable selection of funds. Even actively managed funds can be fairly passive, so read up on the prospectus and find a lowish cost fund that targets an index, even if it’s actively managed.