If I buy a new 16" macbook will Apple transfer from my old iMact to new macbook pro while I wait? Also

We are end users like you, so it is best to ask at the store you plan to use. Services may depend on staffing and other factors, and you will be buying at their busiest time of year.

Because a new Mac will come with macOS 10.15 "Catalina" and Catalina does not support 32-bit apps, true mirroring from an older Mac may not be possible. This is Apple's support article on migrating:

How to move your content to a new Mac - Apple Support

rhart6 wrote:

My iMac is a 2017 so it looks like I can just use Migration Assistant to transfer whatever. Thank you for link.

How to move your content to a new Mac - Apple Support

3-2-1 Backup Strategy: three copies of your data, two different methods, and one offsite.

Boot clone https://discussions.apple.com/docs/DOC-10081

How to use Time Machine to back up or restore your Mac: https://support.apple.com/en-us/HT201250

Use DiskUtility Restore feature https://support.apple.com/guide/disk-utility/restore-a-disk-dskutl14062/mac

If you bring your old Mac or its backup to an Apple-owned store, they generally have a supervised area where they can set you on the right track and answer questions about transferring your data from old device to new. Sometimes they can make adapters available for your one-time use, depending on store policy.

My store was staffed by a group of experts who were eager to help. After an hour or so, they rotated to other assignments and a new group came in, and were just as helpful.

Transferring an entire Mac worth of data can take all afternoon.