Does the latest model version include the old version model?

I'm about to download a model but discovered there are too many versions, are those versions evolving from version to version or are they completely different versions?

If you're talking about models, most of them are mixes, and its recipe can be changed drastically from version to version. Also, changes can be very subtle. Just read "About this version" tab on the right model info box (under "Create" or "Download" button).

Some models can be finetuned. Finutunes are models, that was trained with new images above old model, to change visuals or knowledge about concepts (clothes, poses, character). It can be very similar or different, again.

If you're talking about LoRAs - most of them can be very different as well. From just rebuilding tags in dataset, to expanding dataset further. For example, v1 of LoRA had 200 images in dataset, v2 had 300 images, and v3 would have 500 images in dataset. Of course, output with different versions of this LoRA would be different. Just make sure to read description. (if there is).

In most cases you can just grab latest version of model or LoRA. Just check previews and decide, what you like more.