I'm with you 100% on the importance of immigrants in our country for 100% of the reasons you stated.
To answer your initial question, I'll answer how I personally would define border security. To me, border security is investing in more personnel and more technology (AI, biometrics/facial recognition, transportable surveillance towers, cameras, radar/sensors, etc.) at the southern border. But just as important, ans perhaps more so, it's investing in more immigration system personnel (processing personnel, judges, etc.) to make our system of legal immigration quicker, more efficient, and more humane.
Anyone who thinks we can "close" the southern border or who thinks that 100% of illegal border crossings can be stopped, is delusional. But I do think that there are technological means to beef up border security in a way that isn't performative political theater.