Kurtn is spot on. Starting with the phone off, holding vol+ and power and home will boot the phone into recovery.
He also taught me something - I didn't know about the vol- and vol+ and power to shutdown after flashing! I was popping the battery out after successfully flashing the recovery
No, the klte family doesn't implement that sort of thing, at least not secure-boot-style. There are some specific sub-models that are locked to carriers, but the models in use here (klte, kltechn, klteaio, kltedv, etc) aren't locked.
I had to jump through hoops to unlock my Galaxy Tab A 10.1 (2019) before flashing LineageOS, it ultimately needed a multi-disabler flashed to unlock its bootloader. But Samsung wasn't locking the bootloader like that back in 2014 with the S5. (From memory they introduced locked bootloaders with the S6 or S7.)