My contacts are different on each computer. How can I tidy them up?

I have four computers:

Mac Mini, running Catalina

Macbook One, running Catalina

Macbook Two, running High Sierra

iPhone SE, running 14.0.01

On my Mac Mini, the Contacts are shown as follows:

1) A group of 69 cards "All on My Mac"

2) A group of 31 cards "All Google"

3) A group of 1859 cards "All iCloud"

I don't know why there are these three separate groups. I have always assumed that contacts go into iCloud, and I have never consciously put a contact into Google or 'On My Mac'.

These cards are mostly mutually exclusive, in that if a contact appears in one group, it doesn't appear in another. However, one or two are in both Google and iCloud.

Also, within the iCloud group, there are many subgroups, most of which are duplicated. The contents of both subgroups are the same, but sometimes one group contains more contacts than another.

My MacBook One displays the contacts in the same way as my Mac Mini, except that there are no contacts "on my Mac", except my home contact. Most of the contacts which are on my Mac Mini "On My Mac" do not show up on my Macbook One, which is a big problem.

The subgroups are also duplicated in the same way as the Mac Mini

My MacBook Two only shows 'icloud', with the same list and duplicated subgroups as in MacBook One and Mac Mini.

My iphone also shows only iCloud, with the same contacts as my MacBook Two. And the same duplicated subgroups.

This is a mess, particularly as some important contacts are not on all machines. I have no particular need for Google. How can I fix this?