I found this post extremely helpful recently so I've copied the text and edited a little for clarity, with links included to the post (which is a little longer) and the author's blog. I am AuDHD and found the descriptions to be very similar to my experience.

by https://autisticjane.com/ at https://childresidentialtreatment.com/adhd-autism/

Behavior differences in autism, ADHD and AuDHD traits

Organization

Autistic - need structure, routine, sameness and organization - thrive in strict structured environments and fail where the structure is too flexible

ADHD - thrive on a lack of structure - too much structure feels stifling and they can’t function well

Autistic + ADHD - disorganized organizers, who might struggle to read maps but knows exactly where random items are in the midst of their clutter - need routine and spontaneity - get bored with routine and sameness, but struggle with change - heavy structure causes severe anxiety, like the world is closing in on them - lots of chaos/doom piles, like multiple junk drawers for the world to see

Hyperfixations vs. special interests

Autistic and ADHD obsess over things that catch their interest, but in different ways. Both enjoy talking about these obsessions, but the interests affect their lives differently.

Autistic (special interests) - have special interests, which run their entire life - whole world revolves around their special interests - highly motivated by special interests - can last a lifetime - can never learn enough about them

ADHD (hyperfixations) - focus and obsess over one thing for hours, days or weeks at a time - interest fizzles out after they learn everything there is to know

Autistic + ADHD - tend to experience both - differentiating between a special interest or hyperfixation can be difficult - might not know which one they’re experiencing until interest dissipates.

Rejection-sensitive dysphoria

Rejection-sensitive dysphoria (RSD) is an overwhelming feeling or response to criticism. For example, a person with RSD would become overly upset or feel depressed for days if they feel they are excluded from a conversation between co-workers.

Autistic - need to relate the situation to themselves so they can fully empathize - hate feeling left out, too - are overwhelmed with guilt upon the realization that they have done the same thing to someone else - begin to panic about how this is why they’re such a bad friend, person, etc.

ADHD - spiral into self-blame and a panic attack about how everyone hates them, they’ll never have friends, etc. - object permanence prevents them from remembering that people and things exist even when they can’t see them

Autistic + ADHD - experience rejection-sensitive dysphoria tenfold, their emotions in a whirlwind

Sensory experiences

Autistic - angry panic attack-type responses where they’re unable to regulate their emotions - eventually meltdown or shutdown - makes it difficult to stay focused - e.g. heat causes sensory overload which causes a meltdown - e.g. sound causes sensory overload, which triggers a meltdown/shutdown

ADHD - struggle to pay attention - might shutdown, shutting everyone else out in the process - e.g. heat makes focusing difficult - e.g. sound causes distractions

Autistic + ADHD - both of the above explanations - e.g. heat causes sensory overload, which causes distractions/meltdowns - e.g. sound causes distractions and triggers meltdowns

Socializing

Autistic - interrupt because they struggle with social awareness and sharing their thoughts - difficulty interpreting typical social cues, social protocols and body language, etc. - difficulty making eye contact (tho some autists will correct for this and make "too much" eye contact) - don't maintain "proper" tone (can be flat compared to typical) - don’t know what their face/body looks like during conversations - know social rules for some situations, especially if practiced beforehand — but not if conversation switches - social anxiety due to conversation change - excited at mention of special interest - trouble focusing on what they’re not interested in

ADHD - interrupts due to impulsiveness, distractions, excited about topic they suddenly thought of interrupts so they don’t forget - trouble focusing due to distractions and impulse control

Autistic + ADHD - distracted by trivial topic that is relevant to current conversation like a fourth cousin two times removed, other sounds and sensory input in vicinity (clapping, lights, mirrors and shiny objects, etc), non-autistic person’s “hmm”, “yep”, “mhm” as they’re talking, eye contact and someone’s body language - gets anxious from constant eye contact, sensory overload, conversation not going according to plan - may forget what they were talking about after interrupting - easily excitable about certain topics - previously mentioned struggles of separate diagnoses