How to combine compression with mod_deflate and 206 Partial content in Apache?

Question

PHP

The php script reads the file and echoes it.
The following headers are sent before echo with header.

Date: $date_gmt
Expires: $expires_gmt
Last-Modified: $date_gmt

Further, if a part of the file is requested, then it is sent

206 Partial content
Content-Range: bytes $startpos-$endpos/$fsize
Content-Length: $partsize
Accept-Ranges: bytes
Content-Type: image/svg+xml
Cache-Control: max-age=86400, must-revalidate

otherwise

200 OK
Content-Length: $fsize
Accept-Ranges: bytes
Content-Type: image/svg+xml
Cache-Control: max-age=86400, must-revalidate

Due to mod_deflate, the server adds headers

Vary: Accept-Encoding
Content-Encoding: gzip
Transfer-Encoding: chunked

This removes the Content-Length header.
Will this work correctly? How to check the response when requesting a part of a file?
How does the compression protocol work in general? Are parts of the source file compressed, after which the client unpacks them and appends them to the file, or does the client request parts of the whole archive, and then unpack the whole archive?
Maybe you should turn off compression for image/svg+xml and somehow compress the file using php before sending?