re: Poker & Poker calculators
Yes HUDs are very popular, I'd expect the majority of advanced online players use them, especially those that multi-table.
If the video you watched was one of Marty Smith's training videos, he's a shill for Hold'em Indicator and Tournament Indicator, so all of his videos will have one or the other. These are good for learning how to calculate odds
and outs, and do provide some basic stat tracking like VPIP/PFR/AF and M numbers, but in no way compete with the likes of a full blown tracker like HEM or PT3. Personally I prefer HEM over PT3.
As for cost, none of these are truly free. Both Hold'em Indicator and Tournament Indicator have free trials for like 48 hours, HEM has a 15-day free trial, and I believe PT3 still has a 60-day trial. However all of them also offer free versions through a poker room affiliate signup, where you have to clear so many raked hands to get your free license key. My recommendation would be to avoid this route and signup for rakeback promos instead, which will more than pay for your licenses -- once you join a room through another affiliate link, you can never get rakeback. That's a lesson I learned the hard way when starting out.