Here are some commands for Adb and Fastboot commands. One of the best way to learn new adb and fastboot commands is with the help command. just type fastboot help or adb help to get a working list of commands and variables. See below pictures.
DO NOT USE ANY COMMANDS THAT YOU AREN’T SURE OF OR DON’T KNOW WHAT THEY DO. I AM NOT RESPONSIBLE FOR ANYTHING THAT YOU DO TO YOUR PHONE.

ADB Help

ADB Examples

WHAT IS A LOGCAT

Logcat is the command to view the internal logs of the Android system. Viewing logs is often the best way to diagnose a problem, and is required for many issues. This way you’ll find out what apps are doing in the background without you noticing.

TAKING AN LOGCAT

Now type this in the terminaladb devices (note) to make sure devices is connected

adb logcat -v long > logcat.txt

adb logcat > logcat.txt

you shall have a log file called name of problem.txt inside your ADB folder.  Now open the logcat.txt file and see whats your problem.

Fastboot Help

Examples This is just the start there are tons more and these only work on some devices mainly Motorola and Pixel.

fastboot flash recovery TWRP-recovery.img
fastboot getvar emmc_clr_writeprot
fastboot getvar emmc_set_writeprot
fastboot getvar ptable
fastboot getvar cmd
fastboot getvar resize
fastboot getvar ddrid
fastboot getvar emmcid
fastboot getvar memory_num
fastboot getvar serialno
fastboot getvar product
fastboot getvar VerifyPartition
fastboot getvar nve
fastboot getvar call
fastboot getvar memory_info_
fastboot getvar emmc
fastboot getvar memory
fastboot getvar version
fastboot getvar rescue_version
fastboot getvar rescue_phoneinfo
fastboot getvar vendorcountry
fastboot getvar rescue_ugs_port
fastboot getvar rescue_enter_recovery
fastboot getvar max-download-size
fastboot getvar error_print
fastboot getvar partition-type rescue_recovery
fastboot reboot
fastboot reboot-bootloader
fastboot set_active -a
fastboot oem reboot-download
fastboot oem reboot-ftm
fastboot oem rebootRUU
fastboot dump
fastboot oem listpartition
fastboot oem listram
fastboot oem dmesg
fastboot oem last_dmesg
fastboot oem update_emmc_partition
fastboot oem read_mmc
fastboot oem write_mmc
fastboot oem batt_enable_fast_charge
fastboot oem batt_test_pwr_supply
fastboot oem batt_for_pa_testv
fastboot oem batt_disable_safty_timer
fastboot oem batt_keep_charge_on
fastboot oem uart
fastboot oem htc_radio_debug_func
fastboot oem htcramdump
fastboot oem autordump2storage
fastboot oem test_emmc
fastboot oem ufs_get_lun
fastboot oem ufs_set_lun
fastboot oem erase_phone_storage
fastboot oem unlock
fastboot oem lock
fastboot flashing unlock
fastboot flashing lock
fastboot flashing lock_critical
fastboot flashing unlock_critical
fastboot flashing get_unlock_ability
fastboot flashing get_unlock_bootloader_nonce
fastboot flashing unlock_bootloader
fastboot flashing lock_bootloader
fastboot oem device-info
fastboot oem show-barcodes
fastboot oem ramdump
fastboot oem getcolorid
fastboot oem setcolorid
fastboot oem getcid
fastboot oem setcid
fastboot oem enable-charger-screen
fastboot oem disable-charger-screen
fastboot oem off-mode-charge
fastboot oem select-display-panel
fastboot oem readconfig
fastboot oem writeconfig
fastboot oem easydump
fastboot oem readunlock
fastboot oem dsir
fastboot oem ddrtest
fastboot oem dump_ram_full
fastboot oem get_ks_token
fastboot oem get_anti_theft_status
fastboot oem sha1sum
fastboot oem readmeid
fastboot oem refurbish
fastboot reboot-bootloader
fastboot flashing unlock
fastboot oem unlock-go
fastboot oem frp-unlock
fastboot oem frp-erase
fastboot oem enable reduced-version
fastboot flashing lock
fastboot oem device-info.preflash
fastboot flashing get_unlock_ability
fastboot oem enable-charger-screen
fastboot oem disable-charger-screen
fastboot oem enable-bp-tools
fastboot oem disable-bp-tools
fastboot oem enable-hw-factory
fastboot oem disable-hw-factory
fastboot oem select-display-panel
fastboot oem off-mode-charge enable
fastboot oem off-mode-charge disable
fastboot oem ramdump enable
fastboot oem ramdump disable
fastboot oem uart enable
fastboot oem uart disable
fastboot oem hwdog certify begin
fastboot flash:slock
fastboot oem hwdog certify close
fastboot oem get-imei1
fastboot oem get-meid
fastboot oem get-sn
fastboot oem get-bsn
fastboot oem get_verify_boot_status
fastboot oem enable_dm_verity
fastboot reboot emergency = EDL Mode