i never played these games before. what are they like? FF like gameplay? what are they about? what are the sub series (like frontier, romancing, etc) where should i start?
They are the continuation of the train of thought that went into FF2.
That means no level ups. You gain stats after combat at random, depending on what sort of combat style you used during the fight.
You learn new abilities for your weapon type by attacking strong enemies. WIth some luck you get a "spark", which is a small lightbulb showing up over you head, and you use a new attack free of charge. Afterwards the attack gets added to your moveset, and costs AP to use. Most attacks spark of the basic attack, some however need special spark lines (triple stab sparks of double stab and so on)
Usually the games have level scaling for random encounters, but not for bosses. So most random mobs won't be terribly threatening, unless your hidden power metric (Battle Rank) reaches a new height and they get bumped up a tier.
Also the healing economy is very different. You usually get healed to full after every encounter, and hp is cheap. Bosses can frequently oneshot your units. But normal heals can revive characters. There is no aoe healing, or it is at a very bad rate. There is a second currency of HP called LP which can in some games lead to permadeath upon running out, but usually just prevents you from being revived in combat.
To sum up how all of this stuff plays in combat: Very swingy and fast fights, where unexpected stuff happens constantly and you always have to plan ahead for a level of risk. Also tons of hidden information in everything you do. Sparking system alone is very satisfying, so that if a game contains sparking (like Alliance Alive for example) it is usually called a SaGa like just for that.
Outside of combat the games tend to be very very nonlinear, have just as much hidden information and a fair bit of weirdness to them. Kawazu's mission statement is to make a JRPG closer to crpgs - Every choice matters and you can never know everything about the game and keep discovering new stuff wherever you go. The games tend to be fairly hard if you don't use a guide as a consequence to all the hidden information, but the longer you play the more you understand them, and you get better at them.
Old main series is called FF Legend 1-3. Those are sometimes called SaGa I-III as they were not called Final Fantasy in Japan. Not a very good starting point, as they do not have ability sparking yet. They have a very bonkers setting, with magical worlds connected by towers and stuff, and unit race plays a huge role. Humans, Espers (Mutants), Mechs and Monsters play very differently from each other.
After that there is Romancing SaGa. The title is based on a random shlocky adventure movie Kawazu has seen once. These feature a fairly classical fantasy world with some low level magitech stuff, not unlike Final Fantasy. Romancing SaGa II and III are good entry points here, as they are very easy to get due to the remasters.
Romancing SaGa II is a very interesting game, as you play an entire line of emperors reincarnating all abilites of the predecessors over centuries fighting 7 former heroes, now turned demonic villains. It is a good starting game, because it is a game with ability sparking that does not have multiple starting characters yet, but it can also be a bit tedious since you need to remake your entire build after each generation swap, something which will happen about 20 times and takes 10-30 minutes each time. I started here and did not regret it.
Romancing Saga III is all around a very safe starting point. You can choose the main character of the game out of 8 different heroes, which modifies the story and mostly your starting scenario and end point. Some starting characters make the game much harder, but that also gives it insanely high replayablity. This mechanic has become a mainstay in SaGa.
SaGa Frontier takes those established mechanics of Romancing Saga (multiple starting characters, ability sparking, nonlinear story) and fuses them with the race mechanic of FF Legend 1-3 (Humans, Espers, Mechs, Monsters). It also has a super crazy setting chock full with anime and other culture references, and a very interesting artstyle. If you can stand the radiant weirdness it makes a strong starting point, although you will have to learn much at once.
Saga Scarlet Grace is the newest standalone SaGa game, and all around pretty good. Nothing wrong with starting here, has all the SaGa tropes as far as I know. I havn't played it yet. New world with no or little ties to previous worlds.
There is no real reason to start with SaGa Frontier II or Romancing Saga I at the moment, since the stories between games is not continous, and those games have remasters announced. Since the SaGa revival is the personal pet project of one of the higher ups at Square those all tend to be high quality.
There is no real reason to play Unlimited SaGa not only because there is a remaster pending, but also because the game is considered the worst SaGa.
TL DR:
SaGa games are nonlinear JRPGs with high exploration, good combat, level scaling, much hidden information and many unorthodox mechanics.
Start with Romancing Saga II, III, Saga Frontier I or SaGa Scarlet Grace. You can't really go wrong between those.
Play FF Legend I-III later or wait for Square to port a better version than the gameboy version.
Play the other games when they get their remasters.
These games are generally considered to be some of the best JRPGs in the market and they deserve their reputation.