Attacker using root or SSh to hack my devices how do I remove them?

All of my devices are compromised by an attacker that I believe is living of the land by staying stealthy and gaining root access to my systems or using ssh. If this is indeed the case l, how were they able to gain access and what do I need to do to remove them and keep them out. Any advice into locking down my Windows laptop and Android device will be most appreciated. Thanks in advance.