How can I take a backup of all my emails and mailboxes in apple mail email client?

Use Time Machine or a clone.
Time Machine Basics

Clone - Carbon Copy Cloner (Often recommended as it has more features than some others)
Clone - Data Backup
Clone - Deja Vu
Clone - SuperDuper
Clone - Synk
Clone Software - 6 Applications Tested
Time Machine Versus Clones and Archives
Commonly Used Backup Methods

Or Use Mail Backup for Mac (you can backup all your email clients including apple mail)

Try ZOOK MBOX to PST Converter and instantly convert multiple MBOX files to PST format in a single process. It is the straightforward solution to export MBOX to PST format along with entire data items. The tool facilitates multiple advance features to user by which they can easily convert MBOX to PST format without losing any data. It is an adorable solution for users to export MBOX to PST with attachments and other data items in just two simple steps. In the free demo of the utility, users are allowed to convert 25 items of MBOX file into PST file format for Outlook. For performing the unlimited file conversion, users need to upgrade their tool.

Explore More: https://www.zooksoftware.com/mbox-to-pst/

Download all emails from yahoo server to laptop or hard drive easily with Yahoo backup Application this backup utility developed for windows operating system to take all backup from yahoo server to PST, EML, EMLX, and MBOX file extensions.
Visit here: https://www.datavare.com/software/yahoo-backup-expert.html

There are multiple backup software is available to take Yahoo backup. You need to choose the right one to take all your emails backup from Yahoo server to local drive. I know few utilities for windows that can download all your emails from Yahoo server to Outlook, PST, EML, EMLX, MSG and MBOX file format. If you want to take a backup from Yahoo to a local hard drive or computer.
Visit here: https://www.softakensoftware.com/yahoo-backup.html

Hiii,
You can take backup of you emails by Exporting Mailbox at your selected location in your computer or laptop.
You just need to right click on Mailbox at right sidebar then select Export Mailbox, you can see the displayed image at below. Then choose location where you want to export in hard drive and click choose. After sometime you all email from selected mailbox will be exported at selected destination and it will not erase any email. All your emails will work as same as earlier and you have created a backup of your mailbox too.