I have been working with LIDAR data for quite some time now, starting back in 2011. These techniques are not likely to be useful for you however
https://forums.gpsfiledepot.com/index.php?topic=2579.0This map is the end result of many years playing around with similar techniques. It was not easy to make, the result is low resolution and although it works well in Basecamp and Mapsource, it's too complex to work properly on most garmin GPS units.
https://boydsmaps.com/lidar-in-the-pines-sd/This (and other lidar-based maps on my site) are more like what you want to do:
https://online.boydsmaps.com/#15/39.7929/-74.9632/lidarHDI have Garmin-compatible, downloadable versions on my other site, for example:
https://boydsmaps.com/lidar-in-the-pines-hd/I'm currently building a new site for exploring LIDAR in three dimensions using WebGL.... so I'm all about working with LIDAR.

But (sorry) none of this really addresses your questions. The link that Red90 posted will allow you to make very small, simplistic maps, and it might be a good place to start, but is not really going to do what you want on a large scale. It is limited to making maps that are only 1024x1024 pixels. The unfortunate truth is, Garmin doesn't want you to make big maps like this, because they could compete with their own products.
What you want is called "raster imagery" - maps that are created from pictures of some sort, such as aerial imagery, scanned paper maps, etc. Here's a related thread that discusses some of the techniques and software.
https://forums.gpsfiledepot.com/index.php?topic=4556.0Basically, you need some software that can read the LIDAR data and render it in the style you want. I use GlobalMapper, which is rather expensive. But qGIS is free and can do many of the same things. There are some other programs in the thread above that are more user-friendly (although less versatile).
After you have created the raster imagery, it will need to be converted to .kmz map tiles that are no larger than 1024x1024 pixels each. Different Garmin devices have different limits for how many of these you can use. My Montana can use a maximum of 500. There's some confusion about the 66 series since it's new. Was just discussing this with someone who heard that it can only use 100 tiles at a time, but you can have an unlimited number of maps like this. No confirmation as to whether that is true.
But regardless, it's a very limited format that's inferior to Birdseye in many ways. There's software that will allow you to create your own Birdseye imagery however, but it gets complicated. You need to have an active Birdseye subscription for this to work (which I think is already included with your device) and then you need to jump through a bunch of hoops to trick the GPS into thinking your map is "real" Birdseye.
I don't have the time to get into all the details, sorry. But will try to answer specific questions once you get started.