This is why I got involved with community service; an activity through which I was able to build houses...

Leaving everything on the field was indispensable; an ideology we sixteen individuals adopted...

(are the semicolons being used correctly? if no, what should i edit?)

Just use commas, ie

This is why I got involved with community service, an activity through which I was able to build houses...

Leaving everything on the field was indispensable, an ideology we sixteen individuals adopted...

Or else make what follows the semicolon an independent clause, eg

This is why I got involved with community service; it was an activity through which I was able to build houses...

Leaving everything on the field was indispensable; this was an ideology we sixteen individuals adopted...

But  with this approach, I suggest a period instead of a semicolon.

Best wishes, Clive