There are two sources. Some channels carry a caption track baked into the video stream at the source, the same idea as closed captions on broadcast TV, and your player app just needs to detect and display it. Other content relies on a sidecar file, usually a .srt or .vtt text file with timestamps, that the app loads separately and overlays on the video as it plays. Live channels almost always depend on the first method, because there is no paused file to attach a separate subtitle file to while the stream is running. If a channel's source never included a caption track, no app setting will make one appear.

Whether a channel carries subtitles is decided at the source, by whoever encodes and sends that channel's feed, not by your account or your app. One channel's encoder might include a caption track; the next channel's encoder might not, even inside the same package or the same category of channels. That's why subtitles can look totally normal on some channels and be simply unavailable on others, with nothing wrong on your end and nothing to fix in your settings. It's the same reason picture quality can differ channel to channel even on one connection: the source, not the viewer, sets what's included in the feed.

TiviMate, IPTV Smarters, Kodi and VLC all include a subtitle or closed-caption icon in the player controls, usually a speech-bubble or "CC" symbol you tap during playback. Opening it shows a list of available tracks for that specific stream. If the list is empty or the icon is grayed out, the channel simply isn't carrying a track right now, and there's nothing to switch on. This is a per-channel setting, not a global one, so checking a handful of channels in your app tells you more than checking one and assuming the rest behave the same way.

For downloaded or local video, yes. VLC and Kodi will pick up a .srt file automatically if it's named to match the video file, or you can load one manually from the player menu. For a live channel, that option doesn't really exist, because the stream keeps moving and there's no fixed file to attach a separate subtitle track to in sync. Live subtitle support is limited to whatever the channel's source already embeds. On-demand titles, where the file is fixed and sitting on a server, are the case where adding your own file actually works.

No. Subtitle tracks are plain text with timestamps, a tiny fraction of the data a video stream uses. 4K video runs about 15-25 Mbps and roughly 7 GB an hour on its own; a subtitle track adds a negligible amount on top of that and doesn't touch picture quality. If subtitles look out of sync, that's a timing mismatch in the track itself, or between the file and the exact video it's paired with, not a bandwidth problem. Re-syncing the offset in the player, or switching to an embedded track where one exists, usually fixes it faster than changing anything about your connection.