The lockout in the video looks like its broken to me. Could you feel any difference in resistance in the fork when pressing down (difference between when the fork was 'locked out' or not)? The lockout's function is to save energy by keeping your fork rigid whilst climbing/pedalling 'out of the saddle'. Therefore there should be a significant change in resistance when trying to push the fork down. In my experience you shouldn't be able to push the fork down more than 1-2 cm max when locked out.
The main things that are worse on a cheap are (imo):
-weight (cheaper bikes are heavier)
-supension
-tires
-brakes
-drivetrain
-wheels
Cheaper bikes are heavier in general, mountainbikes are no exception. If you need to climb a lot of hills you will definitely notice the extra weight. Also the more the bike weighs the more weight you will have to accelerate (more watts needed for same acceleration ). When you get into the real expensive segment (multiple thousand dollar for a bike) the weight savings when going for a more expensive model will be minimal (100's of grams) and non racers will probably not notice/need the difference in practice. If you are planning on downhill riding weight is less of an issue (assuming you have a lift to get up the hill). On the cheaper end of the spectrum a price increase will save you more weight per dollar. It is said rotational weight (wheels) is especially important. That being said wheels are quite easy to upgrade after buying a bike.
The suspension quality of cheaper mountainbikes is alright for city use but if you plan on doing anything more demanding and on using the bike a lot it will pay off to have better suspension.
Cheap suspension will not properly smooth out the bumps and will instead transfer a big part of them to your hands via the handlebars which will be very uncomfortable and can cause pain in the hands (personal experience with suntour xct fork riding down. A flight of approx 50 stairs, the same applies to any bumpy trail you plan on riding on with relatively high speed). The cheaper suspension will usually have less adjustment options. The cheaper suspension will be heavier. When going with real cheap coil spring type forks they will bottom out quite easily and the bottoming out won't be nearly as smooth as with an oil dampener (since these really cheap spring shocks essentially use rubber blocks for dampening the impact). The cheaper forks usually won't have remote lockout installed (although it might be available as an upgrade), remote lockout is very useful so you don't have to take your hands off the handlebars or even stop in order to lock/unlock the fork. More expensive forks will have tighter tolerances/less play and will be stiffer. Some more expensive options such as fox float have a system which adjusts the dampening automatically depending on how bumpy the terrain Is you're currently riding over. Some downhill forks will have a triple crown (stansion extend all the way up to the top of the headtube). These forks are stiffer/stronger but also heavier.
Tires on cheap mountainbikes can have a harder compound resulting in much less grip which in my personal experience has caused me to fall whilst cornering on tarmac without rain without touching the brakes whilst not leaning over all that much (maybe 5 degrees lean angle). More expensive tires have better puncture protection in general, also folding type tires cost more and they are a big advantage especially on mountainbikes IMO since you will probably get a few flats and folding tires are in my experience a lot easier to remove/install (not to mention taking a spare tire with you; folding tires take up less space).
Brakes will in general be worse. Cheaper means less adjustment; not being able to adjust the lever positions/bite point, less braking force. Worse cooling )although tbh you will only really need this when riding fast downhill for long sections and braking hard. Some cheaper mtb's will come with cable actuated disc brakes instead of hydraulic ones which is easier in terms of maintenance/spare parts but the hydraulic variant has more precise actuation (you can more precisely determine how hard you want to brake).
Your drivetrain will have less gears when going cheaper. With modern mountainbikes you might get a '1-by' setup where you have only one gear in the front and for example a 12 speed cassette in the rear. Personally its not my favourite due to the cost/availability of replacement parts.
In my experience (on a regular setup with a derailleur both in the front and in the back) the rear derailleur/shifters are the most important parts of the drivetrain since you'll be shifter the rear derailleur a lot more than the front. If you're planning on buying a bike with a somewhat lower end drivetrain and upgrading some of the drivetrain parts later I would suggest upgrading rear derailleur/shifters first. I would suggest deore xt for rear derailleur and shifters and deore for the rest of the drivetrain (IMO deore has a good price/quality ratio). As long as the cables are in good condition and its properly adjusted a deore derialleur will work very will in my experience. Imo an 8x3 speed setup provides plenty of gear rations. Also 8 speed casettes are widely available, easy to find and cheap.
Cheaper wheels are heavier, spokes will often be worse quality (same goes for spoke nipples), rims won't have reinforcing eyelets on the spoke holes, wheel bearings will not last as long. You might get quick release instead of through axle on some really cheap models but I believe that would be quite uncommon nowadays. Through axle is stiffer, stronger, safer.
In general bearing quality will be lower. The frame will also most likely suffer in terms of weight.
When buying a bike from decathlon you might need to check everything after buying to make sure its properly assembled/adjusted. I've seen quite a few bikes there that had issues due to improper assembly/adjustment. The bikes are not bad perse but expect to do a bit of adjustment/fine tuning yourself.
700$ for a full suspension is indeed not a lot. I would suggest a bike twice the price would be a better investment in the long run (depending on your use case). You could perhaps also consider a second hand full suspension since you will be able to find a decent quality full suspension second hand for 700$.
