You can use jq '.[] | .login, .id' to obtain each login followed by its id.

Do it in jq, but see @Kusalananda's answer first

jq -r '.host_components[].HostRoles.host_name | join(",")'

No, that's wrong. This is what you need:

jq -r '.host_components | map(.HostRoles.host_name) | join(",")'

Demo:

jq -r '.host_components | map(.HostRoles.host_name) | join(",")' <<DATA
{"host_components":[
  {"HostRoles":{"host_name":"one"}},
  {"HostRoles":{"host_name":"two"}},
  {"HostRoles":{"host_name":"three"}}
]}
DATA

outputs

one,two,three

Here is an approach using some sample code and data from my answer to a similar question on Stack Overflow.

To choose multiple fields you can use Object Construction. E.g this filter makes an object containing just PublicDnsName and VpcId from each instance:

  .Reservations[]
| .Instances[]
| {PublicDnsName, VpcId}

If this filter is in filter.jq and the sample data from that other answer is in data.json then running

$ jq -M -f filter.jq data.json

produces

{
  "PublicDnsName": "xxxxxxxx",
  "VpcId": "vpc-eb09eb8e"
}

Once you have objects containing what you want getting the data into another format (e.g. csv) is easy. With this filter

  .Reservations[]
| .Instances[]
| {PublicDnsName, VpcId}
| [.[]]
| @csv

and the -r option

$ jq -M -r -f filter.jq data.json

jq produces

"xxxxxxxx","vpc-eb09eb8e"

To add a filtering condition, e.g. VpcId is not null, add a select.

  .Reservations[]
| .Instances[]
| {PublicDnsName, VpcId}
| select(.VpcId != null)
| [.[]]
| @csv

You want to run a .context,.score filter on each element of v I think:

$ jq -r '.[] | [.c, .e, .score, (.v[] | .context,.score)] | @csv' file.json
"A","B",0.99,"asdf",0.98,"bcdfd",0.97

This is equivalent to using the builtin map function without assembling the results back into an array.

I recommend using String Interpolation:

jq '.users[] | "\(.first) \(.last)"'

We are piping down the result of .users[] to generate the string ".first .last" using string interpolation. \(foo) syntax is used for string interpolation in jq. So, for the above example, it becomes "Stevie Wonder" (".users[].first .users[].second" working elementwise) and "Michael Jackson".

jq reference: String interpolation