The a few issues going on here and that YouTube post conflates a few of the technical issues as well.
1. How much internal space a person should buy
2. How much space a person needs
3. Filling a drive
4. macOS's support for TRIM on non-Apple (e.g. external drives) storage
The YouTube poster is correct that Apple's support for TRIM on non-Apple storage is not universal and extensive use of a SSD unsupported by TRIM can lead to severe performance and other problems like he mentioned. However, the issue is not USB-C (a connector standard) but macOS's support for some hardware and some hardware's incompatibility with macOS (especially Monterey+).
One of the reasons why I do not recommend Samsung 980 - 990 SSD is they do something with regards to TRIM that macOS stopped liking in Monterey+. Basically TRIM never finishes and in severe cases you can end up with the situation that the YouTube poster had. This happens for those drives connected via TB3+ / USB4+. Samsung 970 EVO+ (with the final firmware) and earlier did not have that issue and so in some sense better. This issue may have been corrected in Samsung 9100 Pro and later but I can't confirm either way. As a safer (but older) bet for NVMe, I recommend the WD Black SN850X.
In addition to NVMe connected via TB3+ / USB4+ (which excepting above generally support TRIM), some USB3 drives that support UASP may also support macOS's use of TRIM. This may require use of the "trimforce" command from the Terminal prompt (assuming this still works under the latest macOS). The Samsung T5/T7 are known not to work:
I am looking for an external SSD / 40Gpbs Enclosure primarily to use with macos but after researching a lot I am still confused about the current status of TRIM. 1) If macos (Sequoia) does not support TRIM over USB then how does Samsung (T series) or other brands external SSDs support this...
forums.macrumors.com
To be clear, macOS supports TRIM on all Apple's internal SSD (going back at least as far as any hardware officially supported by supported versions of macOS). TRIM is a non-issue with Apple (branded) SSD.
Continuing in reverse order: running a device full is generally a bad idea whether an HDD or an SSD though for different reasons. Though SSD have more issues (made worse if no TRIM support as above), common filesystems such as HFS+ and APFS are designed around having free space to work with and performance gets worse if they are run full for significant amounts of time. Generally it leads to filesystem / layout fragmentation which can be especially painful with HDD. The issue is more severe with APFS on HDD.
Note filling up an otherwise working device once or twice or whatever doesn't cause permanent damage and file system fragmentation can be resolved in the worst case by simply creating a new filesystem (e.g. backing everything up, erasing the device/creating a new filesystem, and restoring the backup). Note I don't consider an SSD without working TRIM to be a working device.
Then as a general rule one should aim to use about 80% or less of a device. In other words aim for total storage 125% of projected total need. Most likely total storage more than that will be needed given that target is on a per device/filesystem basis and those who intend to buy multiple external storage in addition to multiple should round up. As an example if I am buying an external SSD and I currently have 1 TB now that I expect to double over 5 years (the typical target life for an SSD), I would buy 4 TB. 2TB * 1.25 = 2.5 TB -> roundup to 4TB to avoid being at 100% (and likely more if I underestimated growth).
As far as what is needed internal to a Mac depends on the their "data management plan" (whether designed or de facto). The most common system usage profiles I see/hear about are:
a) Basically using Mac as a Chromebook
b) Low I/O apps accessing small working sets of data at any given time
c) High I/O apps accessing low TBs of data
d) High I/O apps access many TB of data
For a) 256GB is fine. After all as defined a Chromebook would be fine. That 256GB is just going to be holding the OS that's going to leave plenty of free space for the future versions of macOS.
b) can also be fine with 256GB especially if iCloud Drive is being used and its configured to not use 100% of the drive (not sure this can be done with some hacking with current versions of macOS as I don't use iCloud). In such a configuration the local SSD is basically just a cache of the iCloud Drive. For a lot of people who may need GBs and TBs of documents in archive but just work with a few dozen Office, PDF, etc files at a time, this may be fine.
Skipping to d) people in this range tend to be fairly tech savvy and no need to go into the details here. However, they will most likely end up with some external storage. At which point the question becomes how much internal versus how much external. 256GB may actually be fine if like a) the internal drive is just used for the OS and one way or the other all their data is external.
c) is where I believe most people go back and forth. Keeping all data on the internal is conceptually simpler, very fast, and most expensive. External storage is generally a fraction of the price and can be +/- as fast as internal but requires a scheme to manage. Options include booting off an external drive (some macOS limitations), external home folders (somewhat more technical, somewhat more error prone), manually allocating/assigning specific folders / archives to sit on the external. Depending on what is chosen here, 256GB is easily adequate or completely impractical.
In summary, I recommend approaching the buying decision as:
1) Identify your usage profile
2) Figure out your storage needs from now through ~ 5 years
3) Buy enough storage/capacity to stay 80% or less through that 5 years
4) Only buy devices that support TRIM (e.g. Apple storage or external TB3/USB4+ enclosures with good NVMe)
