How to interpret this ubuntu server 20.04's default installtion's lsblk output

You are right about /boot/efi. The bootloader for UEFI mode resides there, but if you boot in BIOS mode (alias CSM alias legacy mode), you do not need it. I think that the installer creates it anyway. This is a new 'feature' of 20.04.x LTS, we did not see it in previous versions (at least not previous LTS versions) installed in BIOS mode.

In order to be sure about the boot mode, you can run

test -d /sys/firmware/efi && echo efi || echo bios

The 1K partition is an extended partition, a workaround used in MSDOS partition tables in order to work around the limit of 4 [primary] partitions. You can have several logical partitions inside the extended partition. The size reported here is only the size of the address. Partition #5 is a logical partition inside the extended partition, and inside it there is an LVM structure with the root file system and swap.