I have found out that there are various ways to read boot entries in Linux:
ls -l /dev/disk/by-uuid/
sudo blkid
or supplemented to "sudo blkid /dev/sda"
lsblk -lf /dev/sda
Or only for UEFI systems:
sudo efibootmgr -v
Do all these commands lead to the same result, that I can see the uuid of a special bootloader? When to use which command?
These are not boot entries, but identifiers assigned to a particular file system — UUID stands for Universally Unique ID. When a drive is formatted into one or more partitions, you need to create a file system for each of the partitions. For each of these, a new identifier is created. That's what you're seeing with the blkid and similar commands.
Which command to use depends on what you want to achieve. A lot of commands output similar or related information, use whichever gets the job done faster.