They will start with a hearing test, the SALT team, and take it from there.
To answer your questions, OP, 2 of mine had significant delays. DS1 didn't speak properly until about 4. No hearing issues. Neurotypical. But I was so incredibly worried. He made a huge leap in the summer before reception. I find kids make these massive developmental leaps in the early years, NT and ASD alike.
My youngest also had significant speech delay. The difference with him was that he was a bit 'spacey', sometimes seeming to be elsewhere... there was a little invisible veil over him. My eldest was very clued into people, very sociable and engaging. His speech was not great, but he was very outgoing and babbled a lot. He didn't speak properly but he was heard! My youngest was and is the sweetest boy, sweet as honey but he was such a quiet baby- unusually quiet- as in, you'd forget he was there quiet. He didn't quite connect with people. With me? Yes. Others? Not so much. He was very calm though. He was just extremely focused on flowers, blades of grass, shadows of jets on the green, the movement of clouds above. He was a very, very happy, cuddly baby, just a love sponge, but a bit 'dreamy', didn't respond to his name, didn't point. He was diagnosed as autistic. Fast forward to now. He is 6 going on 7 here and he is an absolute chatterbox. He has an amazing vocabulary and he's very good at taking turns talking.
Typically, ASD kids can get on their soapbox and talk about their exclusive interests. They can have trouble engaging in conversation because chatting tends to be one-sided. But we worked a lot with DS. He used PEC cards to learn the names of things but didn't really need them beyond age 3. I've used PEC cards a lot at the special school I used to work at. They helped older kids who needed to visualise what they were doing 'now, next, after lunch, this afternoon' (i.e. first we are going to the playground picture of swingset, then we area going to the sensory area picture of beanbag. Now it is lunchtime picture of lunchbox with food set out, etc.). My own DS didn't need that level of input from the PEC cards. But they did help increase his vocabulary.
Group speech therapy was brilliant for DS. Between nursery's input and group speech therapy, I believe this was the making of our boy. It taught him how to 'share' talking with others, turn-taking, listening, responding, engaging- this is harder for people on the spectrum. What's important is learning about dialogue and conversation, the social aspect of speech and language. Unless you have a child who really struggles in a group setting and is stressed by this, group speech and language therapy can be very beneficial for neurotypical and ASD children with speech delay (when he is in nursery- around age 3 would be better).
At any rate, you're getting plugged into SALT and that is the right step to take. Speech and language delays are incredibly common, OP and more often, they are nothing at all to worry about. Your little one will be in the right hands. Best of luck!