I'm a grizzled veteran of three critique groups, the longest one temporarily paused at 26 years because we can't do it digitally while including everyone. (One member has no internet at home.)
The ideal critique group is genre-focused, all of them reasonably well-read in the genre even if they don't currently write in that genre. They need to understand what's been done, what's normal or typical, what's too different to sell. A critique group usually sees a fairly small number of pages at a time. My group's limit was 5000 words, and we preferred 3000 or fewer.
A good critique group will not look solely at the writing--although they'll note every mistake and ambiguity, needless words, repetition, and other goofs and weaknesses--but at the concept, character development, plot points, the whole package. They'll tell you what's working so you can build on it and what you really need to shore up before it fails completely.
A proper critique takes time, going over the work line by line. It's an incredibly slow way to go through a novel, although it can be done. Some of my critique group's members occasionally agree to take on a novel outside the scope of the group, but over time we've found that the faults and flaws of the opening chapters are repeated throughout (unless the author has applied what was pointed out to the rest of the manuscript).
A beta reader is the last person to see and comment on the work before it's submitted (or self-published). The author has already whipped it into shape, corrected all errors, edited and revised, and believes they have a polished manuscript ready to go out into the world. It's been critiqued, if it's going to be critiqued, early on, and the author has done their best to use that feedback on the whole.
In times during which meeting in person isn't happening, you can seek or form a critique group online at places like MeetUp, Craigslist, NextDoor, library bulletin boards, and such, with the intention of meeting in person when that is again safe. In my experience, face to face critique tends to be more fruitful than online groups, especially once trust and professionalism among the group is well established. My group even closed itself to people wanting to join us unless they're willing to sit in to observe a meeting and give us a 500 word sample of their writing; we became advanced enough that we didn't want to devote the necessary time to instructing someone who was making a whole lot of basic mistakes or could not handle criticism.
You can, of course, get solid critique here at the appropriate SYW board, but it's not what works for entire novels.
Maryn, who could go on and on